PURPOSE: To simplify structure and to improve reliability by controlling high- pressure liquid without using any large float by providing a float for detecting the liquid surface level on the low liquid surface side and operating a ring mechanism corresponding to its vertical displacement.

CONSTITUTION: When the water level of a paddy field rises, a float 4 rises and a first control link 50 is oscillated upward but when the water level of the paddy field reaches the peak water level, the first control link 50 kicks a lock plate 63 so that the engagement with lock is canceled. At the same time, a second control link 51 opens a first control valve 54 and closes a second control valve 56. Therefore, the supply of water to the paddy field is cut off. On the other hand, when the water level of the paddy field is lowered gradually from a water filling state, the float 4 falls down and when this float 4 reaches a standard water level, the first control link 50 is oscillated downward corresponding to the falling of the float 4. Then, the second control link 51 closes the first control valve 54 and opens the second control valve 56. Therefore, a water supply port 22 is opened and water is supplied to the paddy field.