Official timekeeping in Japan began during the reign of Emperor Tenji (668-672) at his capital Omi Otsu no Miya, with the building of a water clock “to chime the hour with a bell and drum”. The Rokoku did not have an integral bell and drum, but alternatively provided the timing for the priests to do so.

Today, a clock festival (Rokokusai) is held on June 10 each year, and the day is called “Time Day” and is known throughout Japan.

The principle of the clock centres around the lowest tank which is slowly filled from a tank above. As it fills, a rising floating arrow indicates the time against a scale. However, the rate of fill from the tank above is not only dependent on the size of the drainage hole, but the pressure of the water behind the hole. To ensure constant pressure this tank had to remain full. In turn, to accomplish this, there needed to be at least one resevoir above – in the case of the Omi-Jingu Rokoku two tanks above.

A replica Rokoku water clock was donated to the Omi Jingu Shrine, Otsu, and located next to the Museum, by Omega, the Swiss watch manufacturer in 1964.