PURPOSE: To detect a heated state of a food by a simple construction and obtain a specified finish of cooking, by installing a steam port different from an exhaust port in a position where it does not face the air of a cooling fan, and mounting a piezoelectric component in such a manner that it may face sand steam port.

CONSTITUTION: A part of cooled air in a radio wave ejector 8 is introduced into a heating chamber 6 by way of a duct 10 by driving a cooling fan 9. The air 13 which includes a cooled air 11 and vapor or oil generated from a food, is sent to the outside from the heating chamber 6 passing through an exhaust port 14. A piezoelectric sensor 1 is mounted in a vapor port 15 in order to detect vapor generated from a food as its main service. The position of the vapor port 15 is selected so that it may be exposed to a boiling steam most frequently, avoiding the contact with the air of the cooling fan 9. This construction makes it possible to reduce the effect of the cooling air 11 in the radio wage ejector and obtain an extremely stabilized finish of cooking, since only boiling steam is collided with the piezoelectric sensor, thereby providing an excellent output of S/N ratio.