To precisely measure the temperatures of temperature measuring parts as many as possible of a piston within a cylinder by designing the arrangement of each coil unit or its drive in temperature measurement by electromagnetic induction.

The timing of carrying a high frequency current from a high frequency oscillator to the transmission coil 11 of each coil unit U is switched according to each RS signal generated in the bottom dead point of a piston P from a rotating angle sensor, whereby the mutual electromagnetic induction interfering effect between mutually adjacent receiving coils 12 can be prevented. Thus, even when each space between the coil units U is narrow, the temperatures of six temperature measuring points of the piston can be precisely measured.