PURPOSE: To keep the level of water automatically at a fixed one by a method in which a float to be operated by interlocking the vertical action of a gate is floated in the float chamber of a water tank adjacent to waterway and quickly moved vertically ahead of the gate in connection with the vertical movement of the float.

CONSTITUTION: A water tank T adjacent to a gate 1 to open or close a waterway C is divided into an upstream static water pond 5, a float chamber 3, and a downstream static water pond 11. The upstream of the gate is led to the pond 5 and the downstream of the gate is led to the pond 11. A float 2 interlocking the vertical movement of the gate 1 is floated in the chamber 3 and a water conveyance chamber is also partly provided. The water in the water conveyance chamber is moved to the chamber 3 with the vertical movement of the float 2 and the water is quickly moved vertically in connection with the vertical movement of the float 2 ahead of the gate 1. The level of upstream of water in the gate 1 can thus be kept almost constantly by unmanned-powerless manner.