The
Shoku Nihongi records that in 741 AD, as the country recovered from a
major smallpox epidemic,
Emperor Shōmu ordered that a monastery and nunnery be established in every
province, the . The area surrounding the ancient Kai Kokubun-ji ruins () contains the ruins of the
provincial capital and the
Ichinomiya Asama Shrine, the
ichinomiya of
Kai Province, and many late
Kofun period burial mounds from the latter half of the 6th century to the 7th century. It is believed that the
Otomo clan, who were the Kai
Kuni no miyatsuko at the time, were involved in the construction. The exact date of the construction of the temple is uncertain, as it is mentioned in historical records only from 938 AD. It was destroyed by fire in 1225, and although rebuilt on a smaller scale, it never regained its former importance and was abandoned in the
Muromachi period. Per
archaeological excavations, the original site extended from 250 meters east-to-west and 300 meters north-to-south, surrounded by an earthen rampart and moat. Within this compound, the layout of buildings was patterned after the temple of
Daian-ji in
Asuka, Nara, with a South Gate in line with the
Kondō, with the two structures connected by a
cloister. The
pagoda was located on the right side of the courtyard thus formed, and the Lecture Hall (along with other supporting buildings) were located to the rear of the Kondō. Only 25 foundation stones were found at the Lecture Hall ruins, and only two foundation stones remain from the Kondō, but 14 large foundation stones remain from the pagoda ruins, and the corridor has left earthworks-like traces. The foundations for the pagoda indicate that it had a base 16.9 meters square and a probable height of 48 meters. Numerous
Tenpyō period roof tiles were recovered from the site. One of the tile patterns is the "soben hachi" (leaf lotus flower pattern) on the eaves round tiles, and the "kinsei karakusa" (balanced arabesque pattern) on the eaves flat tiles. The same type of tiles were excavated from the Kamidoki kilns site (in
Kōfu) located northwest of the Kai Kokubun-ji, as well as at the Kawada kiln located on the opposite bank of the
Fuefuki River. As the early tiles of Kai Kokubun-ji are also found at the Teramoto temple ruins (in Fuefuki), which believed to be of
Baekje origin, it is believed that
Baekje immigrants were involved in the construction of both temples. The temple was subsequently rebuilt by
Takeda Shingen in the
Sengoku period, but the restored temple was located 300 meters southwest of the original site. The original temple site is located approximately 15 minutes by car from
Isawa-Onsen Station on the
JR Tokai Chūō Main Line. File:Kai Kokubunji-ato, enkei-2.jpg|Panoramic view of precincts File:Kai-kokubunji-ato kondou.JPG|Site of the Kondō File:Kai-kokubunji-ato koudou.JPG|Site of the Lecture Hall File:甲斐国分寺跡 塔跡.jpg|Site of the Pagoda File:Kai Kokubunji-ato, toushinso.jpg|Pagoda center stone ==Kai Kokubun-niji==