PROBLEM TO BE SOLVED: To mainly make it possible that a malfunction does not easily occur and maintenance is easy.SOLUTION: The invention relates to a water level adjustment mechanism 2 which includes a water supply pipe 5 for supplying water to a water supply place 1 and a water supply valve 6 provided on the water supply pipe 5. The water level adjustment mechanism is provided with a water head pressure detecting part 41 for detecting a water head pressure p1 in the water supply place 1. The water supply valve 6 serves as a differential pressure on-off valve 42 which is opened/closed by using changes in the water head pressure p2 on the inlet side of the water supply pipe 5 and the water head pressure p1, which is detected by the water head pressure detecting part 41, in the water supply place 1.SELECTED DRAWING: Figure 2
