PURPOSE: To obtain a fiber optic sensor which removes AC noises by executing feedback to make constant the intensity ratio of DC signals of two wavelengths.

CONSTITUTION: A pulse generator 17 generates pulses for driving LEDs 2a and 2b, and alternately supplies to drivers 16a and 16b. At an optical sensor part 8, only the light of a wavelength λ1 from the LED 2a is modulated intensity, while the light of a wavelength λ2 from the LED 2b is not influenced. A light signal output from the sensor part 8 is inputted to sample hold amplifiers 19a and 19b and separated to a signal by the light of the wavelength λ1 and a signal by the light of the wavelength λ2. The separated signals are inputted to a light power control circuit 21, where the driver 16b is fed back to control the amount of the current fed to the LED 2b so that the intensity ratio of DC signals of the wavelengths λ1 and λ2 becomes constant at all times. In this manner, the light signal of the wavelength λ1 and the light signal of the wavelength λ2 are used as a signal light and a reference light, respectively.