To eliminate the influence of the tilt angle of a band-pass filter.

A collimator 45 which emits polarized parallel light, the band- pass filter 1, and the polarization axis rotary mirror are arranged in order. This polarization axis rotary mirror consists of mutually orthogonal reflecting mirrors 2 and 3 and slant at 45° to the axis of rotation of the band-pass filter 1. The plane of polarization of light returning to the band-pass filter 1 is inverted. Transmitted light passing through the band-pass filter 1 is guided to a collimator 5 parallel to the said collimator 4. The polarization characteristics of the band-pass filter 1 are therefore absorbed by employing the polarization axis rotary mirror.