The Nagaoka capital (Nagaokakyō) was constructed in 784 in the Otokuni district of Yamashiro province by order of the late eighth-century sovereign Kanmu. However, after a period of just ten years, the capital was abandoned and Kanmu relocated his court and capital to Heian.

The history of the Nagaoka capital up until 792 was recorded in the closing chapters of the Shoku Nihongi, an official court chronicle presented to Kanmu in 797. The events of the final years of the Nagaoka capital’s existence were covered by the opening chapters of the Nihon kōki. Unfortunately, the first four chapters of this work, which covered the abandonment of the Nagaoka capital and the move to the Heian capital, are lost. Parts of the historical records contained in those missing chapters of the Nihon kōki have been preserved in the Nihon kiryaku, a Heian-period summary of the Rikkokushi; in the Ruijū kokushi, a late ninth-century work in which material drawn from the official histories is arranged by subject; and in the Fusō ryakki, a late Heian-period Buddhist history of events up to 1094. In the Tokugawa period, a further attempt was made at restoring the missing parts of the Nihon kōki in the Nihon isshi.

Because of the lacunae in the historical records, the short period during which the Nagaoka capital was in use, and a lack of physical evidence for the existence of the capital, the Nagaoka capital soon took on the image of a phantom capital or a temporary capital. It was believed that the Nagaoka capital consisted merely of an imperial palace with some necessary urban facilities but was not a fully planned Chinese-style capital as were, for example, the Nara and Heian capitals. As a result, the location of the Nagaoka palace had already become unclear by the Kamakura period. It was only during the Meiji period that the location of the Nagaoka palace gradually became more accurately known again. At the end of the nineteenth century, Yumoto Fumihiko pinpointed the location of Kanmu’s audience hall (daigokuden) in the ‘Daigokuden’ quarter of Kaide in Mukō city. Based on Yumoto’s findings, a stele commemorating the Nagaoka palace was erected on the presumed location of the imperial audience hall on the occasion of the 1100th anniversary of the transfer of the capital to Heian.

Research on the Nagaoka capital, rather than the palace, started only 1908, when the historian Kita Sadakichi published a series of four articles entitled Nagaoka sento kō [Thoughts on the transfer of the Nagaoka capital]. For the first time, the Nagaoka capital was recognized as a real Chinese-style capital consisting of a palace area surrounded on three sides by an urban area. Kita also presumed that its extent was no less than the scale of other capitals such as Nara and Heian.

Another forty-five years passed before an attempt was made at systematically excavating the capital’s remains, partially because researchers assumed that the chances of finding remaining traces were slim, given the short period of time during which the Nagaoka capital had existed. At the instigation of historian Nakayama Shūichi, a resident of Mukō city, the first excavation finally took place at the end of 1954.

— Adapted from the introduction to Ellen Van Goethem, Nagaoka, Japan’s Forgotten Capital