An apparatus for measuring the temperature of a piston in an internal combustion engine. The apparatus includes a thermistor to sense the temperature of the piston and generate an electrical signal representative of that temperature, a transmission unit connected to the thermistor for receiving the electrical signal and for converting the signal to an infrared beam for transmission to a point remote from the piston, and a receiver to receive the beam and convert the beam to an electrical signal corresponding to the electrical signal generated by the thermistor. The first mentioned electrical signal is converted to a rectangular wave form prior to transmission of the infrared beam.