To make it possible to reduce delay in response of a value detected by an exhaust air temperature sensor to changes in actual catalyst temperature, to reduce difference between the actual catalyst temperature and the value detected by the exhaust air temperature sensor, and to detect exhaust air temperature with a high degree of accuracy.

A catalyst for purifying exhaust air is provided in an exhaust air passage in an internal combustion engine. An exhaust air sensor for detecting composition of the exhaust air is provided on the upstream side of the catalyst in the exhaust air passage. The exhaust air temperature sensor for detecting the exhaust air temperature is provided on the downstream side of the catalyst, including the inside of the catalyst, in the exhaust air passage. A heater which generates heat or stops with the power supply being on or off is provided in the exhaust air temperature sensor. And then, control means for controlling switching between on and off of the power supply to the heater, in response to the value of the exhaust air temperature detected by the exhaust air temperature sensor, is provided.