To cut off a high-voltage transformer from a commercial power source in a microwave oven, in case of a short-circuit failure of a power adjusting control switching means, by quickly detecting the failure.

A control circuit 23 judges the presence of short-circuit failure of a control relay switch 14 from the detected state of a voltage detecting circuit 22 in an intermittent mode, and turns off a power supply relay switch 7 in the judgment of the short-circuit failure to cut off the high-voltage transformer 13 from the commercial power source. Accordingly, even if a welded (short-circuit) failure occurs in the control relay switch 14, the continuous carrying of excessive current to the coil of the high-voltage transformer 13 can be prevented. Accordingly, increase in size of the coil of the high-voltage transformer 13 or improvement in cooling effect of the high-voltage transformer 13 is not needed, and the manufacturing cost can be thus reduced. The excessive heating of a cooking matter can be also prevented.