To facilitate the accurate measurement of a spectrum regardless of the polarized state of an incident light.

A polarized state averaging means 1 averages the polarized state of an incident light 11 from a light source 10. A polarizing component selecting means 2 selects a polarized component having a specified polarized wave face from an incident light 12 emitted from the polarized state averaging means 1. A polarized wave face rotating means 3 rotates the polarized wave face of a polarized component 13 selected by the polarizing component selecting means 2 at rotating speed different in each wave length. A separating means 4 separates a specified polarized light 15 from the emitted light 14 of the polarized wave face rotating means 3. A photoelectric converting means 5 converts the strength of the polarized light 15 into an electric signal. A computing means 6 computes the spectrum of the incident light on the basis of the electric signal outputted from the photoelectric converting means 5.