PURPOSE: To reduce cost and obtain a high accuracy by a method wherein a ratio with respect to the set value of an output is accumulated at every one cycle of an AC power source voltage while an ignition command is outputted every time when the value of the accumulation has become higher than the new value which is integer times as large as a predetermined value.

CONSTITUTION: The ON ratio of a set input value 4a from the output value setting circuit 4 of a microcomputer 11 is accumulated through an ON ratio accumulating means 12 at every one cycle of an AC power source voltage (a). Every time when the value of accumulation has become larger than integer times of 100%, an ignition command 13a is given to a cycle pulse producing means 14 through an ignition commanding means 13. An intermittent cycle control function may be realized by a simple operation processing method effected by the microcomputer 11 through a constitution omitting the circuits of a V-F converting and delaying function or the like whereby the cost of control of the cycle of an AC voltage may be reduced. Furthermore, the control means of an AC power under an intermittent cycle control may be made highly accurate by the microcomputer 11.