A polarization separating element such as a beam splitter receives a first light of a first polarization plane. The polarization separating element derives out of the first light a second light of a second polarization plane set perpendicular to the first polarization plane. A photoelectric element converts the second light into an electric signal. The intensity of the second light is in this manner detected. Since the second light is derived out of the first light, the phase of the second light is allowed to synchronize with the phase of the first light. The extinction ratio of the second light accurately reflects the extinction ratio of the first light. The detection of the intensity of the second light at the photoelectric element enables an accurate detection of the intensity of the first light.