PURPOSE: To save labor and optimize the water temperature by providing an opening/closing ring on a float lifted or lowered in response to the water level fluctuation of the paddy field, and automatically opening or closing a conducting tube feeding water from a conduit to the paddy field via the siphon principle with the opening/closing ring.

CONSTITUTION: A float 11 formed with a water drop port 15 at the center is liftably inserted into a cylindrical main body l provided with an opening section 13 and a drain port 14 at the upper and lower sections. A slide bar 22 having a projection 21 at the upper end is protruded above the float 11. A cylindrical ring 2 is freely fitted to the bar 22 so that it can be slidably lifted or lowered in the main body 1 to form a water level adjusting unit A. The adjusting unit A is arranged on the paddy field, a conducting tube 12 is connected to a conduit having a higher water level, and water is fed via an siphon action. The float 11 is lifted to close the conducting tube 12 with the ring 2 at the proper water level, and the ring 2 is lowered at a stretch by the projection 21 interlocked with the descent of the float 11 to feed water when the low water level is attained. The reduction of the water temperature over a long period is prevented, and labor can be sharply reduced.