To provide an optical spectrum analyzer can measure the optical spectrum intensity and the polarization state of input light signal without conforming the polarization direction to the polarization axis of a polarizer by controlling the polarization of the input light signal.

The optical spectrum analyzer includes: a spectroscopic part 12 separating the light input from a light signal input part 11 in response to the wavelength; a Stokes parameter measuring part 13 measuring a Stokes parameter expressing the polarization state of the light separated by the spectroscopic part 12; and a spectroscopic wavelength controlling part 14 sweeping and controlling the wavelength separated by the spectroscopic part 12.