LESS than three years ago, Koshiro Tanaka turned in his dark suit and tie, the uniform of the Japanese office worker, in exchange for a salwar kameez, the pajama-like outfit worn by Afghans. Mr. Tanaka was armed only with his convictions and had had no practical battlefield experience when he joined the mujahideen (resistance fighters) in their war against Soviet forces in Afghanistan.

``I didn't know how to fight, how to move,'' Tanaka says.

But he learned quickly. Accompanying a band of mujahideen on a raid on a government post near Jagdalak, about 25 miles east of Kabul, Tanaka recalls, ``I felt a bullet go by my ear. I got a shot of adrenalin.''

Since then, Tanaka - holder of a sixth-degree black belt in karate - has been educating Afghan guerrillas in the use of the martial art in hand-to-hand combat in a war he has embraced as his own. His motive is simple and hard-line. He wants to fight the Soviets. ``They [the Soviets] don't want peace, they want land,'' Tanaka says.

The Japanese government frowns on Tanaka's activities, and Japan's Embassy in Islamabad, Pakistan, has warned him about becoming embroiled in the Afghan struggle. But he pays no attention.

Tanaka is exporting his skills and enthusiasm to Afghanistan because a military career is not open to him in Japan. The country's pacifist post-World War II Constitution places great restriction on militarization, and Japan has only a small ``defense force.''

Tanaka criticizes the government and the Constitution for denying Japanese the chance to mount a fighting force to drive the Soviets off the Kuril Islands, an island group northeast of Japan which Japan claims as its own by virtue of proximity and historical ties. When the spoils were divvied up at the end of World War II, the Kuril Islands went to the Soviets.

The Afghan Muslim guerrillas fighting their jihad, or holy war, see death as the ticket to martyrdom. While Tanaka doesn't follow the tenets of Islam, his view of death is not much different from that of the Afghans. He sees himself as a potential martyr for Japan. Tanaka sees a passive Japan as an easy target for the Soviet Union in what he terms its grand design for world domination.

Born generations too late, the 46-year-old Tanaka longs for the ancient Japanese samurai traditions. ``I hope in my mind that I will have the samurai spirit when it is time to die.''

His ``death'' has been the subject of much propaganda emanating from Kabul. The Afghan government has twice claimed to have killed him. In the February issue of a government publication, Afghanistan Today, a full-page spread tells of Tanaka's death during an ambush by government troops.

Since his first trip to Afghanistan in 1985, Tanaka says, he has returned six times. His recently published autobiography, ``Soviet Soldiers in a Gun Sight, My Battle in Afghanistan,'' details his exploits with the mujahideen.

Sitting in his cubbyhole of an office in Tokyo's Shinjuku district, Tanaka talks of preparations to go to Afghanistan before the end of the summer. When he returns, he will take radios and cameras. But with the exception of a handful of donations, the money will come out of his own pocket.

When he traveled last year to Peshawar, the capital of Pakistan's Northwest Frontier Province, Tanaka took $10,000 to distribute to the mujahideen. He raises the finances for his activities through a small company he has set up for this purpose. He also says that most of the proceeds from his book will be donated to the mujahideen.

Peshawar is the seat of the Afghan resistance movement. Seven Muslim guerrilla parties that have formed a loose and sometimes fractious alliance have their headquarters there. (An ancient trading center and once the summer home of Afghanistan's royalty, Peshawar is now home to more than 3 million Afghan refugees.)

Tanaka joined up with Jamiat-e Islam, believed to be the second largest of the mujahideen groups, headed by Prof. Burhanuddin Rabbani.

The mujahideen, he says, ``need help, any kind of help. They need weapons, bread, food, anything.'' Most mujahideen weapons are primitive and antiquated, and these are in short supply, Tanaka says.

On his coming trip, Tanaka intends to travel to the Panjshair Valley, northeast of Kabul. It is a three-week hike from the northern Pakistani town of Chitral over the Hindu Kush mountain range. He says he will link up with one of the few Afghan commanders to secure an international reputation, Ahmed Shah Massoud. Mr. Massoud has studied karate, Tanaka says, but ``he's not so good.''

A poster-size picture of the Afghan commander hangs alone in the center of one wall in Tanaka's office. On the other walls, crowding each other for space, are posters in several languages, portraying the Afghans' seven-year struggle to rid their country of Soviet occupation.

Alongside the posters of homeless women and children, Kalashnikov-wielding mujahideen, and victims of bombing raids, is a handful of articles in various languages about Tanaka's exploits in Afghanistan.

Although Japanese right-wing organizations, which should see in Tanaka a kindred spirit, donated money for the Afghan cause last year, he says the source has all but dried up.

Tanaka was once a member of such a group, but now, disillusioned by their lack of support, he says, ``They are just for themselves. It's just a lot of noise.''

A visible and vocal group in Japan, the far-right groups cruise Tokyo streets in mesh-encased vans. A megaphone atop the truck blasts out their slogans, adding to the the city's already overwhelming noise pollution. The Soviet Embassy in Tokyo has been a regular target of their threats and violence. Riot police in armored vehicles sit at either end of the street where the embassy is.

At a recent reception to launch his book, Tanaka regaled a gathering of about 40 with stories of Afghanistan. The audience was a mix of socially prominent Japanese, reporters, and businessmen. All listened as Tanaka railed against the Soviet Union, nodding their heads, applauding occasionally.

As his guests arrived, Tanaka's wife, Takiko, stood beside him, showing her support, but dreading his departure. With the assistance of an interpreter, Mrs. Tanaka admitted she doesn't understand her husband's zeal for the Afghan mujahideen and his answers have left her none the wiser. She says she has tried to stop him, but to no avail.

Two years ago Mrs. Tanaka traveled to Peshawar with her husband. The crowded streets, open sewers, and bazaars were in stark contrast to Tokyo. But she says she felt Peshawar's charm. And when her husband is there, waiting to go inside Afghanistan, her memories of the city bring her a little closer to him.

Will she read his book? There's no hesitation. ``I think it's better for me not to read it.''