To incorporate a calibrating function for correcting an aging change of a thermistor.

A black body of a temperature equal to an environmental temperature is introduced into an overall visual field of a sensor S, and hence a sensor R and the sensor S are placed thermally in the same environments. Thereafter, a difference (Rs-Rr) of a resistance value Rs of the sensor S and a resistance value Rr of the sensor R is detected in step S1 by an operation of switch inputting or the like. Here, if a value of Rs-Rr is '0', it is judged that a calibration is not necessary, and a process is completed (step S2). In step S3, (Rs-Rr) is converted into a temperature Tsr. Then, in step S4, the temperature Tsr is stored as a calibrating parameter δT.