PURPOSE: To stop a draft drainage pump safely and surely and prevent its breakage by stopping the operation of the draft drainage pump when a current detector is operated.

CONSTITUTION: When a starting switch 27 is closed, a contactor 23 is operated, a contact 23ma is closed, a draft drainage pump motor 24 is started, and the water level in a draft pipe 6 is gradually lowered. The water head of a draft drainage pump 8 is made high, and its discharge quantity is gradually decreased. When the water level in the draft pipe 6 is lowered to the position of a screen 9, a current detector 25 detects the shaft power when the discharge quantity of the draft drainage pump 8 becomes the minimum discharge quantity, and its contact 25b is closed to operate a pump stop relay 31. A contact 31b is opened, the contact 23 is returned, the contact 23ma is opened, and the draft drainage pump motor 24 is stopped. When the draft drainage pump motor 24 is operated for a fixed period, an operation time monitoring timer 30 is operated, a contact 30b is opened, and the draft drainage pump motor 24 is stopped.