PURPOSE: To eliminate any erroneous operation by clarifying the difference between abnormal super heating and a normal heating irrespective of variation in temperature of atmosphere and to enable an abnormal super-heating condition to be detected fast when the abnormal super-heating occurs even if the temperature of atmosphere is low.

CONSTITUTION: A magnetron 6 heats an object 90 to be heated within an oven housing 5 through a wave guide 7. A cooling fan 9 sucks surrounding air and cools the magnetron 6. The cooling air is fed from a housing suction port 10 into the oven housing 5. A discharged gas temperature sensor 14 is arranged within the discharging duct 12 so as to judge the presence absence of an abnormal superheating operation condition in response to the discharged gas temperature. An atmosphere temperature sensor 30 for sensing the temperature of atmosphere (surrounding air) is arranged near the suction port 15. The discharging temperature Hτ is corrected for the atmosphere temperature Qτ and if Hτ-(Q'-25)×K1 is higher than a predetermined value, the oscillation of the magnetron 6 is stopped under an assumption that the abnormal super-heating operating condition occurs.