PURPOSE: To obtain a large output response with respect to the minute displacement of a displacing body, by selectively inserting a plural kinds of light transmitting bodies having different refractive indexes or thicknesses between an optical sensor head and the displacing body.

CONSTITUTION: When the light transmitting body 2 is inserted between the displacing body 8 and the sensor head 1, the equivalent optical distance between the displacing body and the head is reduced from l by Δl, with an aperture angle 2θ1 of projected light being constant. Meanwhile, when the light transmitting body is not inserted, the output of a light receiving element is P1. Since it is located in the saturated region of the output, the response width of the output P is small with respect to the minute displacement of the displacing body. On the contrary, when the light transmitting body is inserted, the distance is equivalently reduced, and the output becomes P2. Since this is in the non-saturated region, the large response can be obtained with respect to the minute displacement of the displacing body.