PURPOSE: To make change in water level slow as well as to lower the frequency of operation suspension by obtaining the hourly variation of water level continuously measured, and thereby selecting the combination of respective pumps in such a way that the absolute value of water level shall be the smallest one other than zero when water level is raised.

CONSTITUTION: The water level 3 of a scale settling pond 4 to which roll cooling water and the like 3 and rain water and the like 2 flow in, is measured by a level gauge 12. Measured water level signals are inputted to a differential circuit 14 via a signal conversion circuit 13, they are then inputted to a command circuit 15 which selects the combination of a plurality of drain pumps 5 through 11 so as to direct commands, thereafter, so that the respective pumps 5 through 11 are thereby controlled In this case, the hourly variation of water level continuously measured is obtained. As a result, the combination of the respective pumps 5 through 11 is so selected that the absolute value of the hourly variation of water level shall be the smallest one other than zero. By this constitution, the basin can be drained in such a way that the water level of the scale settling pond 4 is kept at a point between the highest water level and the lowest one.