PURPOSE: To measure body heat in a short time in combination with an output of a pyroelectric element by interrupting infrared rays entering the element via a condenser lens with a chopper allowed to measure the rays.

CONSTITUTION: A rectangular case body 10 is provided with a pyroelectric element 11, a chopper 14 having a thermistor 15, a motor 16 for driving it, an arithmetic section 18 and a display section 13. A cylinder part 12 is provided at one end open part to hold an infrared condenser lens 17 to introduce incident infrared rays. The chopper 14 is rotated by the motor 16 and internal oral temperature is calculated with the arithmetic section 18 from an output signal from the element 11 and the temperature of the chopper from the thermistor 15 to be shown on the display section 13. The opening is provided at the tip of the cylinder part 12 sticking out of the case. An incident range of the infrared rays into the element 11 is fixed to facilitate the holding of the cylinder part 12 in the mouth. Thus enables the measuring of an oral part in a short time thereby preventing mistake such as measuring the temperature around the mouth or of teeth or the like.