An optical modulator is hung from a key moved on a key trajectory so as to be moved together with the key, and a light beam, which is stationary with respect to a key bed, is radiated through the optical modulator; a large number of miniature refractors are formed on the light output surface of the optical modulator, and the density of miniature refractors per unit area is varied in a direction in which the optical modulator is moved together with the key so that the current key position is converted to the amount of light passing through the optical modulator; since the miniature refractors and other portions of optical modulator have a unitary structure, a molding process is used for the optical modulator, and the molding process make the production cost low.