PURPOSE: To reduce load of a driver by providing a current detector operated for generating a current detection signal and a control circuit which opens and closes a minimum flow rate bypass valve upon reception of the current detection signal and an operation signal accompanied with pump operation.

CONSTITUTION: A bypass line 10 on which a minimum flow rate bypasss valve 9 is arranged is branched from a discharge piping 6 and connected to a water source 4. A flow rate meter 12 is arranged on a flow rate measuring orifice 7 for indicating a supplied water amount. A pump operation signal 15 is output from a driving motor 15 accompanied with the operation of a pump 2. A current detector 17 connected to the driving motor is operated for outputting a minimum flow rate current signal 18 when current flowing to the driving motor 1 reached a preset value corresponding to a minimum flow rate of the water supply pump 2. The minimum flow rate current signal 18 and the pump operation signal 15 are inputted to a bypass valve control circuit 19 where the minimum flow rate bypass valve 9 is opened and closed by the signals 18 and 15.