PURPOSE: To stably control a water level for a distributing reservoir accurately corresponding to the substantial fluctuation of a water distribution amount flowing out from the distributing reservoir.

CONSTITUTION: An opening degree signal based on the detection of the opening degree of a flow valve 4 provided on the inflow side of the distributing reservoir 1 and a water level signal based on the detection of the water level WL of the distributing reservoir 1 are inputted to a controller 7 for the inflow valve. The controller 7 performs flow rate deviation control in which the ratio of the present water distribution amount obtained from the product of water level change per unit time and the area of the distributing reservoir to the reference water distribution amount of the distributing reservoir 1 is defined as a deviation when the water level WL of the distributing reservoir 1 is within a fixed allowable fluctuation range A for a set value WLo, and performs water level control when the water level WL of the distributing reservoir 1 is within an abnormal fluctuation range B exceeding the allowable fluctuation range A.