A microwave oven has a case forming a cooking chamber, a door for opening/closing said chamber, a high voltage transformer for generating a high voltage, and a magnetron for generating microwaves driven by the high voltage outputted from the high voltage transformer. The microwave oven further includes a microswitch serving as a detector for detecting opening/closing of the cooking chamber; a control device for outputting a driving signal corresponding to the detected opening/closing of the chamber, and an opening/closing device for controlling a power source supplying power to the high voltage transformer according to reception of the driving signal.