An optical spectrum analyzer includes a diffraction-grating control unit configured to change an angle of a diffraction grating to change a wavelength of a dispersed light beam extracted from incident light, a calculator unit configured to calculate an angle of the diffraction grating such that the wavelength of the dispersed light beam has a sampling wavelength, and to store the data indicating the angle, a FIFO memory configured such that part of the data is inputted to it, for outputting the data at each reception of a trigger signal indicating timing of sampling, and an FIFO memory control unit configured to output the subsequent data to the FIFO memory, when a remaining data amount of the FIFO memory reaches a predetermined value or lower.