It was here in the rainforests of the Owen Stanley Range that he would return, much older, to honour a grim promise that would take him 26 years and 400 million yen in life savings and pension payments to carry out. "I can never forget that pledge to my comrades," recalled the 88-year-old veteran, whose failing strength has forced him to abandon his one-man mission and return to Tokyo. "It was January 12 [1943], and our food had run out. By then I weighed less than 30 kilograms and, like the other troops, I was eating the flesh of dead enemy soldiers just to stay alive.

"Those who were strong enough were evacuating from the coast, deserting the weak and ordering them to keep the Australians and Americans at bay. So I said to the soldiers left behind: 'No matter what happens, if you die in this land I will come back for you, and I'll return you to Japan to rest with your families. This is my promise to you'." Such was the old soldier's determination to make good on his word that he walked out on his wife and two sons in 1979 to do so. As he reflects in a book to be released next week - The Bone Man of Kokoda, by the Australian journalist Charles Happell - he did not stop to think about them once in the 2 decades he subsequently devoted to digging up and repatriating the remains of almost 350 Japanese soldiers. "Why waste thoughts on something like that?" he told the Herald from his home north of Tokyo. "I don't know if they're even alive any more. They didn't approve of what I was doing, and nor did the rest of Japan. But I gave a pledge. How could I sit here in Tokyo while my comrades were lying forgotten beneath the dirt, so far away from the families that grieved for them every day?"

So 37 years after the troops in his battalion became the first members of the Japanese army to reach New Guinea, the 60-year-old retired mechanical engineer made his way back. This time he was armed with a landmine detector, a mattock and a shovel. His solo quest consumed almost a third of his life - but he says he would still be digging if he had the strength.

With his savings he bought a hectare of land at Kakandetta, between Giruwa on the coast and Kokoda inland, built himself a two-storey house out of local hardwoods, and planted a vegetable plot beside it to supplement his simple diet. Then he set to work. "I felt like this was my life mission," explained the proud nationalist, who says his family crest links him directly to Japan's imperial family and the Shinto gods who fashioned Japan from the elements. The Australians and Americans had shown respect for their war dead, he said. Their remains were carefully interred at cemeteries across the countryside. The Japanese government, on the other hand, had "literally left thousands of Japanese soldiers to rot, as if they were an embarrassment that they preferred to forget about. So I took it on as my own duty. No one else would."

Not least among his objectives was a yearning to recover the bodies of close friends from his own platoon, all 55 of whom were killed on the Kokoda Track. Most of these soldiers, from Kochi or Ehime on the island of Shikoku, were killed by Australian machineguns in the Battle of Brigade Hill, on September 8, 1942. Seventeen days later, the Japanese retreated. It was a turning point in the Pacific War. Despite being shot three times in the shoulder from almost point blank range during the battle, Nishimura survived - even chasing and killing his assailant, an Australian soldier who "looked to me just like a teenager". "Every day for many, many years I could hear the scream he made when I ran my sword through him. It was a horrible thing," he recalls.

On his return, he took his old diary and detailed notes he had smuggled out - against strict military orders - in the evacuation. By using those to supplement his fading memory of the year he spent in tropical hell, he mapped out land he thought most likely to yield the remains of fallen comrades. First he searched for deviations in the earth that looked like the imprints of old foxholes - small pits dug by soldiers to hide themselves in battle. He would test the ground for mines, then use a stake to gauge how soft the soil was and therefore how likely it was to conceal decomposed human remains. Finally, Happell writes in his account of Nishimura's mission, he would go to work with his shovel and garden hoe removing sand and soil that had built up over 40 years. "The bodies were rarely buried deeper than a metre. Often, soldiers just dropped where they were shot; occasionally they were spreadeagled on top of each other."

At Giruwa beach he found 120 bodies and at Buna and Gona, two small coastal towns not far away, 60 skeletons. At Waju he recovered a further 30 to 35 bodies. After just a few years of digging, his house became an ossuary for the forgotten Japanese soldiers of New Guinea. As his work took him further afield, Nishimura was saddened to discover that in several places opportunists had assembled Japanese skulls and war effects in morbid displays to attract passing tourists. The Japanese government knew of this but had done nothing to stop it.

A Japanese professor of humanities, Utsumi Aiko, on a recent visit to the Indonesian island of Biak, was equally appalled to discover the amount of unclaimed Japanese remains being shown by small-time entrepreneurs. Of the 2.4 million Japanese military dead, she wrote, "1,160,000 of them are still not repatriated. With close to half of the dead in overseas battle zones, we greeted the 50th, then the 60th year after the war. "Even now some 600,000, it is said, are retrievable. Why in the world don't we retrieve them? Why aren't the bereaved families urgently concerned about the bones of their relatives?"

Nishimura was sure the families did care. But he knew most felt helpless to do anything without government assistance. That was where he planned to make a difference. Occasionally dog tags, dental work and other unusual markers let him identify remains. Bones he could not identify he hoped to one day submit for DNA analysis - a fledgling science in the middle years of his work.

On frequent trips back to Japan, Nishimura would spend days tracking down relatives of soldiers whose effects he had found. "I can't describe the feeling I had at seeing tears of relief. They were finally reunited with the sons who had never come home from war," he said. Among the haul was a lunch box he had inscribed for his training instructor, Lieutenant Yoshiyuki Morimoto, 40 years earlier, which he had dug up in an empty field. As the maverick crusader's reputation spread in Japan, he received more requests for help from families to recover their loved ones. One man, Kokichi Morimoto, travelled to New Guinea to search for his lost father Toshio and, when excavating a vegetable field, uncovered the remains of more than 100 Japanese soldiers. "The skeletons appearing from the soil just broke my heart," Morimoto said afterwards. "Just imagine how it must have been to be abandoned for so many years in the middle of nowhere so far away from home."

It was not until 1988 that Nishimura finally returned to Efogi, the site of the Battle of Brigade Hill, where his battalion had been wiped out in some of the fiercest fighting of the Pacific War. With the help of curious locals, he spent several days digging the sticky, red Kokoda Track battleground.

"Finally I struck black soil and then I found some charred bones nearby. Then I realised what had happened; the Australians had burnt hundreds of bodies to stop the unbearable stench," he says. "It was devastating. This had been a special mission for me. All I could do was to take some of the ash away in tins. Now it is at the Gokoku Shrine at Kochi, so at least I know my old friends are resting safely back at their home with their families." In 1994, Japan's ambassador to Papua New Guinea, Tadashi Masui, told Nishimura that while his government appreciated his work, it wanted him to hand over his collection of bones as a symbolic gesture to mark the 50th anniversary of the end of the war the next year. Nishimura agreed, only because he felt the ambassador implicitly assured him Japan would make every effort to identify the remains, which amounted to more than 200 bodies. Instead it cremated and interred them at Chidorigafuchi Cemetery in Tokyo.

"Throughout all these years, there were many setbacks," Nishimura said. "But the biggest obstacles were put in my way by Japan, which showed either neglect for the thousands of young men who died in its name, or deceit. It brought tears to my eyes to think that my country could do this." Eventually, after he became ill and returned to Tokyo in late 2005, the government cremated the rest of the skeletons he had retrieved and sent the ashes to Chidorigafuchi.

As Happell sees it, many Australians will still feel the Japanese were a "barbaric enemy, and they were. They had an uncommon bloodlust and their treatment of POWs was appalling. I'm not trying to dress them up as anything else. But, in Nishimura, they may possibly find a Japanese soldier to empathise with. If nothing else, he certainly took the concept of 'mateship' to another level." The Japanese lost about 13,000 soldiers on the Kokoda Track and the beaches to the north. That was more than three times the number of casualties suffered by the Allies, who lost 3095 Australian and American soldiers between them. But monuments to the Japanese dead are few and most are in disrepair. So in 1989, Nishimura erected his own at Efogi - to all soldiers killed at New Guinea. The stone, in Japanese kanji characters inscribed by monks from Tokyo's Zenshoan Temple, reads: "To The Loyal War Dead."

Strangely, it was this tribute that prompted the Kochi-New Guinea Association of war veterans to eject Nishimura from its ranks, for acting without authority. As his lifelong friend and fellow New Guinea veteran Sadashige Imanishi said before his death last year: "We don't tolerate unique people very well in Japan." Nor does Nishimura have much time for Japan: he feels the country that he loved has lost its soul, and is "a country in name only". Foreigners might see a nation with a profound sense of its own distinctive culture, he said, but that impression was "merely a facade".

Young Japanese thought more about Western fashion, phones and iPods than their own history: "Soon I will be gone, and others like me too. Who will be left to remember?" The Bone Man Of Kokoda, by Charles Happell (Macmillan Australia, $32.95) is in bookstores from Tuesday.