PURPOSE: To prevent the overload on a pump by opening/closing a delivery valve to the predetermined opening in accordance to the water level through the function of a water level switch which functions when the water level at the suction side varies by the predetermined amount.

CONSTITUTION: A water level switch 24 which functions when a suction well 4 is at the predetermined level and an intermediate opening limit switch 25 for opening/closing delivery valves 21aW23a fixed to pumps 21W23 which functions upon the function of said switch 24 are provided. When the water level in the well 4 lifts to reach to the switch functioning water level 8" a closing command is provided to a delivery valve controller 26 from a signal indicating that it has exceeded over the water level 8, while the switch 25 functions to operate the controller to the valve closing thus to squeeze the delivery valves 21aW 23a to the intermediate opening. When the water level drops from above the operating level 8" to below said level, the switch 24 will function to provide the open command from the controller 26 thus to open the delivery valves 21aW23a.