PURPOSE: To permit the idle revolution speed control with high precision by controlling the generated electric power of an alternator according to the control instruction value which is formed by adding the correction instruction value with the average generated electric power control instruction value for holding the battery voltage at a prescribed value, when idle revolution is detected.

CONSTITUTION: An alternator 14 all-water-rectifies the three-phase ac generated in a stator coil 18 by a rectifier 20 and supplied said electric current into a battery 12 and an electric load 10. In this case, one edge of the rotor coil 16 of the alternator 14 and the anode of a diode 17 are connected with the collector of a transistor 1 which is turned-ON/OFF according to the output of the pulse width modulation circuit 22 of an exciting current setting means 24. When the idle state is detected by an idle state detecting means 40, the output of an adder 30 which adds the signal after the integration processing 32 of the output of a battery voltage comparing circuit 28 and a revolution speed correction instruction signal supplied from a deviation detecting circuit 36 is selected by a selecting switch 26 and input into the exciting current setting means 24.