PURPOSE: To achieve a higher speed and a higher accuracy of the temperature measurement with the lowering of voltage by performing preheating while emplying non-linear characteristic with a P-N junction.

CONSTITUTION: A sensor tip 11 directly contacting body heat comprises a circuit made up of transistors Tr1WTr3 and resistances R1WR3 to obtain a reference voltage Vret stably and a diode D1 as non-linear preheating element. An A/D converting section 16 applies the output of an operational amplifier 14 to an arithmetic processing section 17 and based on the results of the processing, a display section 18 indicates the temperature of body heat. In this case, the preheating is done utilizing non-linear current voltage characteristic based on the P-N reverse characteristic of the D1.