PURPOSE: To prevent the overflow of water from a regulating reservoir by providing an effluent amount arithmetical circuit for effluent from a steam power plant, a forecast water level arithmetical unit for a multi-purpose regulating reservoir, and a dam gate opening and closing command circuit for regulating reservoir.

CONSTITUTION: When a steam power plant 5 begins to operate, used water is discharged and stored in a regulating reservoir 8, and when a gate 6 for spillway is opened, the water of a reservoir 3 is discharged into river 1 and stored in the regulating reservoir 8. An effluent amount 29 as the total sum of the effluent amount 22 os the power plant 5 and an effluent amount 23 from the gate 6 for spillway by opening operation is put in a dam gate opening and closing command circuit 30 by an effluent amount arithmetical unit 21, and in the unit 30, the number of gates 9 to be opened is calculated by dividing the effluent amount 29 by the effluent amount when one of the gates 9 of reservoir dam is fully opened. By a forecast water level signal 28 from the forecast water level arithmetical circuit 26 of the regulating reservoir 8, an opening and closing operation command 31 including the gate number is sent out to a field control board to open or close the gates 9.