PURPOSE: To maintain revolution of an induction motor constant irrespective of a variation in a powre source voltage.

CONSTITUTION: A full-wave rectified voltage of a commercial AC power source 1 is compared with a DC constant voltage by a comparator 12, and a signal of a full-wave rectified waveform interval width Wp is output to a microcomputer 13. The microcomputer 13 corrects a gate pulse generating basic phase angle θ to be output from a revolution switching circuit 14 in response to the magnitude of the interval width of the output of the above comparator, and outputs it to a driver 15. The driver triggers a traic 17 based on it, and controls to drive an induction motor 18 at a predetermined revolution. Even if a power source voltage is varied, the revolution is not changed by a programming in the microcomputer but a stable rotation control can be conducted.