PURPOSE: To remove drift of an output signal due to temperature fluctuation and to make a sensor part compact and light weight, by performing the feedback control of frequency of light from a light source means based on the output of the interferometer of the sensor part.

CONSTITUTION: The output of a laser light source 21 is transmitted to a Fabry- Perot interference sensor part 46 through a modulator 41, a splitter 42, and an optical fiber 44. The light is reflected and interfered between semitransparent films 23a and 23b. The amount of the reflected light to the fiber 44 is sent to a detector 26 and detected as the change in physical quantity at the outside. The detected output is compared 28 with a reference voltage V and made to be a control signal (c) through an LPF29, which extracts only the drift of a low frequency component, and an integrating circuit 30. Based on the signal (c), a VCO47 feeds back a light frequency shift signal (f) to the modulator 41. As a result, the phase shift caused by temperature fluctuation in the sensor part is compensated and the temperature drift is removed.