PURPOSE: To carry out a stable relay operation by constituting a screw pump as a relay pump in such a way that when the load of the motor of the screw pump becomes less than a prescribed value, the pump is stopped, and when the water level reaches a prescribed level or above, the pump is started.

CONSTITUTION: A load detector is provided to the electric motor of a screw pump as a relay pump, and when the load drops lower than a prescribed value, the motor is stopped; and also a water level detector is provided for detecting the water level, and when the water level rises above a prescribed level, the motor is started. Thus, the inflow water quantity can be accurately relayed, and the malfunction due to a mixture can be prevented.