PURPOSE: To secure the safety of high-voltage circuit components by suppressing the level of the output set signal to the preset value or below until the input current to an inverter circuit exceeds a fixed value when high-frequency induction heating is started.

CONSTITUTION: The input current to an inverter circuit 30 is detected by a current transformer 80, the DC voltage with the level corresponding to it is fed to a microcomputer MC 62 from a bridge rectifier 81. The MC 62 calculates the average value of r.m.s. value of the input current to the circuit 30 from the output of the rectifier 81 and increases the level of the output set signal when it is smaller than the preset value to increase the on period of a power transistor Tr35. The MC 62 decreases the level of the set signal when it is larger than the preset value to decrease the on period of the power transistor Tr35. When the on period of the Tr35 is increased, the high-frequency current flowing through a heating coil 50 or the primary coil 40a of the driving transformer of a magnetron 44 is increased, and the output is increased. When the on period is decreased, the output is decreased. No overload is thereby applied to elements of the circuit, the life of the elements is improved, and the safety of high-voltage circuit components can be secured.