PURPOSE: To prevent noise and to improve sensitivity, by selecting the wavelengths of the output light beams of two light sources so that the light beams can be received by Si light receiving elements and the output of the divided value from an operating means is approximately linearly changed with respect to the change in temperature.

CONSTITUTION: A first light source 10 has a wavelength λ1 at the center of light emittance. A second light source 11 has a wavelength λ2 at the center of light emittance, which is different from the wavelength λ1. An operation circuit 17 holds the amount of received light corresponding to, e.g., the first light source 10, and performs the dividing process of said amount and the amount of the received light corresponding to the second light source 11. The light from the source 10 is inputted at first and the light from the source 11 is inputted later. A voltage proportional to the quotient is outputted. The output voltage of the operation circuit 17 can be approximately linearly changed with respect to the change in ambient temperature of a semiconductor material 6 by the selection of the wavelengths of the light source 10 and the light source 11.