Asuka Mizuochi Site

Japan's oldest water clock

This is the site of Japan's oldest water-clock tower, built in 660. It lies on flat land along the Asuka River at the eastern foot of Amakashi Hill.

The Nihon Shoki (Japan's oldest official chronicle, compiled in 720) records that in 660 Prince Naka-no-Ōe built a rokoku (water clock) and for the first time announced the hours to the people. This site is thought to be the one described in that entry.

What excavation revealed

A stone-faced platform surrounded by a moat, with a building on top whose floor was supported by a full grid of pillars. Wooden pipes ran inside the platform, and a lacquered wooden box was set at its centre.

The structure was unlike anything found before, but investigation showed it to be a water-clock tower for telling the time. Water flowed from a supply tank at the top down to a receiving tank at the bottom, and the time was read from a scale as the water level rose. The clock is thought to have used tanks arranged in steps.

The mechanism is thought to have come from Tang China.

Why a clock?

Measuring the time and announcing it to people was an essential part of building a ritsuryō state (a state governed by law codes). Officials' hours of work, the start of ceremonies, the opening and closing of markets: without a shared time, the state could not function.

The water-clock tower was built 15 years after the Taika Reforms. The clock was set in place as part of the process of giving the state its shape.

The clock that moved to Ōtsu

These structures were abandoned in the late 7th century. When the capital moved to the Ōmi Ōtsu Palace, the water clock was probably moved from Asuka to Ōtsu as well.

All that remains on the site today is the stone-faced platform and markers showing where the pillars stood.

Visiting

On 26 July 2026 it was inscribed as a World Heritage Site, as one of the component parts of "Asuka-Fujiwara: Archaeological Sites of Japan's Ancient Capitals and Related Properties."

About 3 minutes on foot from the "Asuka" stop on the Asuka loop bus from Kintetsu Kashiharajingu-mae Station. Admission is free and the site is open at all times.

The Ishigami site is right next door, and Asuka-dera Temple and Amakashi Hill are a short walk away.