PURPOSE: To measure temperature in a living body in safety over a long period, by impressing energy which is emitted by a variable-frequency oscillator at the outside of the living body, to a crystal oscillator which is implanted into the living body, and recognizing a frequency at which said energy is absorbed, and thus detecting the temperature in the living body.

CONSTITUTION: High-frequency energy from a variable-frequency oscillator 5 is radiated by a coil 3, and is transferred to a coil 2, and oscillates a crystal oscillator 1. When the frequency of the oscillator 5 and the resonance frequency of the crystal oscillator 1 coincide with each other, the oscillation of the crystal oscillator 1 becomes large, and absorbs energy. Then, the frequency at which the absorption of energy is caused is read with a frequency counter 6, by varying the frequency of the oscillator 5, under monitoring an energy detector 4. The value which is read by the frequency counter 6, reflects a temperature which the crystal oscillator 1 implanted in a living body receives from the surrounding living body.