An optical sensor includes a light source, a sensing unit, a first optical fiber transmitting light from the light source to the sensing unit, second and third optical fibers transmitting the light from a reflecting surface of the sensing unit to light-receiving portions, and a calculation unit calculating physical quantities from electrical signals from the light-receiving portions. End surfaces of these fibers opposed to the reflecting surface are fixed so that the longitudinal direction thereof and the normal line of the reflecting surface form an angle .theta., the second and third optical fibers are parallel to each other, and the fixing angles of the first optical fiber and the second and third optical fibers are symmetric about the normal line. These optical fibers have a single mode in the wavelength of which the optical fibers are used. A depolarizer is interposed between the light source and the first optical fiber.