A first linearly polarized bundle of rays of incident rays is converted by optical modulation with an input video signal into a second linearly polarized bundle of rays orthogonal to the first rays in polarization. The first rays pass through a polarizer before optically modulated. A second linearly polarized bundle of rays originally involved in the incident rays is reflected by the polarizer in a first direction. The second rays obtained by the optical modulation are reflected by the polarizer in a second direction. The second rays reflected in the second direction pass through another polarizer. A first linearly polarized bundle of rays involved in the reflected second rays is reflected by the other polarizer. The reflected first rays is detected by an optical sensor that is positioned outside an optical path of the second rays originally involved in the incident rays and reflected in the first direction.