PURPOSE: To detect the quality and characteristic of a subject in a highly accurate manner by eliminating a mirror reflection light from a reflected light out of the subject, and assembling a polarizing beam splitter that is able to take out a diffuse reflection light alone.

CONSTITUTION: An irradiating light out of a light source 1 is made incident on a polarizing beam splitter 2, and a rectilinearly polarized light in a specified direction is generated, a subject 3 is irradiated with it. A reflected light out of this subject 3 comprises a diffuse reflected light (dispersed and/or diffused on a surface or inside) showing the quality and characteristic of this subject 3, and a mirror reflection light, not reflecting those of quality and characteristic. The reflected light inclusive of these two components is made incident on the beam splitter again, and only the diffuse reflection light is taken into a spectroscope 4 (or directly to a detector 5), eliminating the mirror reflection light. In succession, this diffuse reflection light is diffracted in a specific wavelength range by the spectroscope 4, and it is converted into an electric signal by the detector 5 or detected directly by this detector 5, thereby converting it into the electric signal. With the mirror reflection light eliminated, high quality information reflected to a characteristic of the subject is thus securable.