A study shows that the keep of Maruoka Castle, seen here in Sakai, Fukui Prefecture, was built during the Kanei Era (1624-1644), and not earlier. (Keibu Horikawa)

SAKAI, Fukui Prefecture--The keep of Maruoka Castle here, long hailed as the oldest in Japan, is anything but, new research shows.

It turns out that the structure previously attributed to the Warring States period (late 15th to late 16th centuries) was probably constructed during the Edo Period (1603-1867).

Sakai municipal authorities, eager to promote the significance of the castle, had maintained on its website and elsewhere that it was the oldest in Japan.

The keep is designated by the central government as an important cultural property.

In seeking to upgrade its status to a national treasure, the city's education board set up an expert panel in fiscal 2015 to study the building. The members had expertise in history, architectural history, archaeology and structural dynamics, among other fields.

The keep’s principal components, such as posts and beams, were studied by using three dating methods: annual tree rings, radiocarbon and oxygen isotope ratios.

The study showed that the timber used for the key parts of the structure, which collapsed during the Fukui Earthquake of 1948, had been felled no earlier than the second half of the 1620s during the Edo Period.

That led the panel to conclude that the keep was built by Honda Narishige, the inaugural lord of the Maruoka feudal domain, during the Kanei Era (1624-1644).

In the past, there were two theories on when the keep was constructed.

According to one version, it was built in 1576 by Shibata Katsutoyo, a nephew of Shibata Katsuie, a chief retainer of the warlord Oda Nobunaga. The other version says it was built after Honda Narishige became lord of the castle in 1613.

According to the Agency for Cultural Affairs, castle keeps built before the Kanei Era include Matsumoto Castle’s Inui Kotenshu (northwest minor keep) in Matsumoto, Nagano Prefecture, which is assumed to have been built in 1592, and Inuyama Castle in Inuyama, Aichi Prefecture, which is believed to have been built in 1601.

That conflicts with assertions that Maruoka Castle’s keep is the oldest in Japan.

“The news is disheartening, all the more so because we were hoping to attract more tourists ahead of the extension of the Hokuriku Shinkansen Line to Fukui Prefecture four years down the road,” said Tatsumi Iwata, head of the city government’s Maruoka Tourist Association. “But that does not devalue Maruoka Castle in the least.”

The study also showed the keep was repaired, after it collapsed in the Fukui Earthquake, with components that had survived since the Edo Period. It was rebuilt on the same scale, structure and style as before the quake.

“I think the construction period has become better defined, and the structure has become academically more significant,” said an official with the Agency for Cultural Affairs, adding that there are no plans to overturn the castle keep’s status as an important cultural property.