PURPOSE: To stably keep the water level in a water distribution reservoir to be a proper preset level while retaining the pressure in the upstream piping higher than the allowable lower limit in which the distributing area in the upstream side is not affected, by regulating a flow rate control valve, taking the pressure in the upstream pipes into consideration and appropriately controlling the inflow volume to the distribution reservoir.

CONSTITUTION: The relationship between the pressure in the upstream pipes 2 and the proper inflow rate into a water distribution reservoir 1 necessary to keep this pressure higher than an allowable lower limit has to be obtained in advance The pressure in the upstream pipes 2 is detected by a pressure gauge 4. The proper inflow rate corresponding to the detected pressure is calculated on the base of the relation and the required opening extent for the proper inflow is calculated and further, the flow rate control valve 3 is operated. And the proper inflow volume is supplied to the distribution reservoir 1 to recover the water level to a preset figure.