PURPOSE: To detect accurately a heating state of a foodstuff by providing a heating chamber, a high frequency supply means an environment detection part and a high frequency detection part.

CONSTITUTION: When a foodstuff 14 placed in a heating chamber 3 is heated with a high frequency supply means (magnetron) 4, vapor is generated from the foodstuff 14 and the vapor is diffused in the heating chamber 3. The vapor diffused in the heating chamber 3 is introduced to a gas passage having an opening 17 in a part of the ceiling surface 13 of the heating chamber 3 with the air in the heating chamber 3 where they come in contact with a board 21 in the upper part of the opening 17 in the passage of the gas and detected with an atmosphere detection part (thermistor) 25 installed to the top of the board 21. A part of the high frequency reflected from the heating chamber 3 is radiated into the gas passage from the opening 17 of the ceiling surface 13 and detected with a high frequency detection part (antenna) 26 installed to the bottom of the board 21 in the upper part of the opening 17. This construction makes it possible to detect accurately a heating state of the foodstuff 14.