In Japan, what is the longest river?

The Shinano River, known as the Chikuma River in its upper reaches, is the longest and widest river in Japan and the third largest by basin area (behind the Tone River and Ishikari River). It is located in northeastern Honshu, rising in the Japanese Alps and flowing generally northeast through Nagano and Niigata Prefectures before emptying into the Sea of Japan.

The Shinano River Project, funded by the British Academy, investigates the development of the Shinano—Chikuma River system. It has uncovered some of the earliest examples of Flame-style pottery (kaen doki), dating from the Middle Jōmon period 3500-2500 BCE), have been found at Sanka excavation site in Nagaoka, Niigata, near the confluence of the Sai and Shinano Rivers.