@ An infrared sensor (4) made of LiTaO 3 is provided on a ceiling of a heating chamber of a microwave oven, and a chopper (5) is provided so as to intermittently block infrared radiation emitted from food (2) in the heating chamber (1). The infrared sensor (4) is brought to face the food (2) and the chopper (5) alternately and provides a corresponding AC output signal. The temperature of the chopper (5) is detected from the output signal of a separately provided chopper temperature sensor (8) consisting of a thermistor. The detected chopper temperature output signal is corrected in a microcomputer to be substantially in accord with chopper temperature components in the output signal of the infrared sensor (4). The corrected chopper temperature data is applied to the infrared sensor output data for removing the chopper temperature components from the output signal of the infrared sensor to obtain a food temperature signal corresponding to the sole temperature of the food (2).