An object is to accurately measure the Stokes parameters, without the occurrence of polarization fluctuations or PDL during the splitting of the incident light. When the incident light is made incident on a first-stage prism, the light is split into two first splitting light rays. Next, the first split light rays are respectively incident on a pair of prisms of a second stage. Each of the pair of first split light rays is split into two rays by a second-stage prism, to obtain four second split light rays.