To obtain an optical apparatus by which a polarization direction can be selected by passing the linearly polarized component turning accompanying change in the phase difference between luminous fluxes of linearly polarized light obtained by passing the multiplexed luminous flux of two linearly polarized luminous fluxes through a quarter-wave plate, through a polarizing plate.

Linearly polarized luminous fluxes LM1, LM2 are reflected and transmitted respectively by a polarization beam splitter 3, and a multiplexed luminous flux is obtained, and the multiplexed luminous flux is transmitted through a quarter-wave plate 43 polarization axis is tilted by 45° with reference to the polarization direction of one of one of the luminous fluxes LM1, LM2. Then, clockwise polarized light TM1 and counterclockwise polarized light TM2 are obtained, they are overlapped so as to obtain linearly polarized light TL which has a polarization face in a prescribed direction. A polarization direction at this time is changed accompanying the rotation of a radiation grating. Then, the polarized light TL is divided into two luminous fluxes TL1, TL2 by a semitransparent mirror 10 so as to be introduced into polarizing plates 11, 11' which are used to select respectively prescribed polarization direction components. The polarizing plates 11, 11' are used to change the luminous fluxes TL1, TL2 into components in a constant vibration direction, and only the components are received by photodetectors 12, 12'.