To enable an optical pressure guage to measure pressure, by arranging a polarizer, a wave plate and an analyzer, in order from the incidence side, penetrating and retaining the wave plate by a strut whose ends are retained by the polarizer and the analyzer, and changing transmission light intensity by the rotation of the wave plate by pressure.

A pressure surface 2 like a phosphor bronze thin plate is formed on the upper surface of a pressure vessel 1, via a movable joint 3 like a bellows. On the optical component base 9 of the bottom part of the vessel 1, a polarizer 5 and an analyzer 7 are arranged on the incident ray side and on the outgoing ray side, respectively. Between the polarizer 5 and the analyzer 7, a 1/2 or 1/4 wave plate 6 is rotatably penetrated and retained by a strut 10 both ends of which are point- retained in recesses on the side surfaces of the polarizer 5 and the analyzer 7. Between the pressure surface 2 and the wave plate 6, a pressure transfer rod 8 for transferring the pressure on the pressure surface 2 to the wave plate 6 is installed. According to the strength of pressure, the wave plate 6 is rotated, the optical axis is changed, and the intensity of linearly polarized light is changed. The polarized light passes the analyzer 7, and the intensity of the transmission light is changed, The change of light intensity is detected by a photodiode via a lens 4 and an optical fiber.