The thermopile and a thermistor, in the manufacturing stage, and variation occurs for each product. The characteristics of the thermopile and a thermistor built into the final product is expected to be different for each finished product. In addition, there is a problem in that it is also influenced by the variation of the assembling the assembling step, when ensure the measurement accuracy of the final product, there can be provided inexpensively. And the light-receiving unit for measuring the relative temperature difference, and the temperature measuring circuit for measuring the temperature of the light-receiving unit, in addition to the relative temperature difference between the each of the monitored area with the temperature of the light-receiving unit, calculating the temperature of each the surveillance region, and outputs the calculation result and it provided with a calculating circuit for, each time the measurement is corrected for the calculated results according to the temperature of the light-receiving unit. Thermopile, a thermistor, a calculation result, the relative temperature difference