PURPOSE: To judge a heating completion timing of food irrespective of the type and weight of food to be heated in response to water vapor generated from the food and the like.

CONSTITUTION: A magnetron 3 radiates a high frequency wave into a heating chamber 1 in response to a driving operation with a control means 4. A thermistor 7 detects water vapor discharged from a discharging port 6 and outputs an electrical signal Va. An AC amplifier circuit 8 outputs an AC component of the electrical signal Va from the thermistor 7 as an AC signal Vb. The control means 4 decreases an output of the magnetron 3 periodically under a driving state of the magnetron 3, and stops a driving of the magnetron 3 when a signal level of the AC signal Vb from the AC amplifier circuit 8 exceeds a set level.