To precisely measure the pressure applied to a pressurizing body part by providing a corner prism and a pressurizing body on the tips of two optical fibers, and measuring the phase difference between the incident light of one optical fiber and the output light of the other optical fiber.
Each of lights having wavelengths λ1, λ2 emitted from lasers 1, 2 is propagated in an optical fiber 3, transmitted by a 45° polarizing element 16, and then inputted to a corner prism 4. The light reflected twice there and outputted from the same surface is transmitted by a pressurizing body 5, and propagated in an optical fiber 6, and the lights having wavelengths λ1, λ2 are separately detected by light receiving elements 7, 8. Since the corner prism 4 is used instead of a conventional 1/4 wavelength plate, the cost can be reduced. Since a pressure change, even when a pressure is added to the whole circumference of the pressurizing body 5, is added only in the transmitting direction of light, the pressure in one direction can be detected. The ratio of outputs of the two wavelength lights is determined, whereby the optical quantity change can be compensated.