PURPOSE: To take a measurement with high precision and high sensitivity without any influence of external variation by using a compound semiconductor for a pressure detecting element, reading light absorption wavelength directly, and converting it into pressure.

CONSTITUTION: Light emitted by a light source 1 is incident on the pressure detecting element 3 through the compound semiconductor through an optical fiber 2 and light transmitted through the element 3 reaches a demultiplexer 9 through an optical fiber 20. The path of the light is divided into two by the demultiplexer 9; one light beam is guided from a spectroscope 10 to a pressure display part 7 and the other light beam is guided to a photodetector 4. Projection light from the light source 1 enters a signal processing part 6 through a photodetector 5 so as to compensate variation of the light source, and signals from the photodetectors 4 and 5 are processed to display the result on a display part 70. Then, the spectroscope 10 is so operated that a signal output Q which varies with measured pressure and is displayed on the display part 70 is constant at any time, thereby selecting wavelength corresponding to the output Q. Thus, variation in pressure is detected by utilizing variation in wavelength, so there is no influence of external variation factors.