PURPOSE: To prevent generation of a large rush current by optimizing an output timing of an ON signal with respect to a relay of a high voltage transformer of a magnetron irrespective of manufacturing tolerance of a relay, fluctuation of a power source voltage, variation of a relay delay time, and a change with the lapse of time.

CONSTITUTION: A primary coil of a high voltage transformer 10 for supplying power to a magnetron 9 is connected to an AC power source 1 via a relay contact (b) of a relay 12. A relay coil B of the relay 12 is excited by an output port 6b of a microcomputer 6 through a transistor TR3. A current transformer 13 is provided for detecting a current on the primary side of the high voltage transformer 10 so as to convert the current into a voltage. The microcomputer 6 measures a relay delay time ΔT of a detected time of the current transformer 13 from an ON time of the transistor TR3, to store it thereinto. Thereafter, the microcomputer 6 outputs an ON signal to the relay 12 at the optimum ON timing calculated on the basis of the relay delay time ΔT.