PURPOSE: To prevent destruction of a magnetron and the loss of resin components due to melting by immediately stopping transmission of electricity to the magnetron if a cooling fan is stopped.

CONSTITUTION: The contacts 7b, 6b of relays 7, 6 are closed in sequence at the press of a keyboard 4 and then transmission of electricity to a magnetron 12, a fan motor 8, an oven lamp 9, and a turn table motor 10 is started so that heating is started. When the fan motor 8 is stopped, generation of voltage at a secondary coil 8b is stopped. The voltage is compared with reference voltage for a reference time. When the voltage generated is lower than the reference voltage for more than the reference time, a control unit 8 immediately opens a relay contact to stop electricity transmission. Destruction of the magnetron and the loss of resin components due to melting are thereby prevented.