PURPOSE: To detect changes in physical quantity accurately with a wavelength little affected by disturbances by arranging a first optical fiber for transmitting light from a broad range luminous source, a second optical fiber for transmitting emitted light from an optical signal converter adapted to emit light only with the wavelength corresponding to displacement of physical quantity such as temperature and pressure among lights emitted and a spectroscope for determining the wavelength of emitted lights.

CONSTITUTION: Light 6 from a first optical fiber 2 is dispersed in various wavelengths with a concave grating 31. When a moving object 34 is moved in the direction of the arrow with the deformation of a diaphragm 32 due to the displacement of a specified physical quantity, a slit 33 formed thereon 34 is positioned corresponding to the displacement if the physical quantity. At this point, a dispersed light 7 from the concave grating 31 passes through the slit 33 thus positioned corresponding to the position of the displacement. This light 7 passing through the slit 33 is introduced into the second optical fiber 4. This allows only light to pass through the slit 33 with only the wavelength corresponding to the displacement of temperature, pressure or the like.