PURPOSE: To measure temperatures, by using two light beams having different wavelength, making both light beams pass the same light transmitting path, varying only the intensity of the light beam with one wavelength in response to a tempeature by a tempeature sensor, and comparing the ratio of the intensities of the split light beams and the quotients before the light beams are inputted to the temperature sensor.

CONSTITUTION: Parts of light beams having wavelengths of λ1 and λ2 are split by a light splitter 54. One group of light beams is inputted to a light source monitoring light receiving element 60 through an optical fiber 56, and the light beams are transduced into electric signals by a signal processing part 61. The other group of the light beams is inputted to a light receiving element 59 through a temperature sensor 57 and an optical fiber 58, and the light beams are transduced into electric signals by the signal processor 61. The electric signals corresponding to the light beams of λ1 and λ2 which are received by the light receiving element 59 are made to be V1λ1 and V1λ2. The electric signals received by the light receiving elements 60 are made to be V2λ1 and V2λ2. Said electric signals are amplified by an amplifier 62, and the division operations of the signals are alternately performed by a divider 45. The results of both divisions are held by a sample and hold amplifiers 44 and 44'. The division operations are performed by a second divider 45' again. The result is amplified by an amplifier 63 and inputted to a display part 64, and the temperature can be displayed. Thus the functuation in the characteristics of the light transmitting path of an optical temperature sensor can be compensated.