There are provided amplifying part (5) for amplifying an output signal of infrared detecting part (4), gain switching part (6) for switching a gain of amplifying part (5) to one of a plurality of gains according to magnitude of the output signal of amplifying part (5), and measuring part (12) for calculating a temperature based on an increase in the output signal of amplifying part (5) relative to an initial reference value, and after writing command input part (11) input a writing command, the output signal of amplifying part (5) is written as the initial reference value for each of the plurality of gains in storage medium (13). Since measuring part (12) uses the initial reference value corresponding to the gain as the initial reference value, temperature of object to be heated (1) can be accurately measured over a wide range.