A spectroscope capable of suppressing the dimension and the cost with avoiding a problem caused by polarization dependency of the diffraction grating. The spectroscope includes a polarizing beam splitter plate 3 that divides the light from an input fiber 1 into a first and a second optical paths and polarizes each light of each optical path having different direction of polarization with each other, a Fresnel rhomb half wave plate 5 that arranges the direction of polarization of the light in the first optical path into that in the second optical path and a plane mirror 4 that deflects the second optical path such that at least a portion of an area where the light through the first optical path is incident on the diffraction grating 6 and an area where the light through the second optical path is incident on the diffraction grating overlap each other.