An optical spectrum analyzer detects a light output that is dependent on the frequency of light in a wavelength range of light to be measured. The optical spectrum analyzer includes a waveguide acousto-optic tunable filter including a piezoelectric substrate, optical waveguides, and an IDT, a light source for providing, to the waveguide acousto-optic tunable filter, reference light having a particular wavelength outside the wavelength range, a driving circuit for providing, to the waveguide acousto-optic tunable filter, a high frequency signal for exciting an IDT, and an arithmetic device that, on the basis of the wavelength of selected light when reference light is incident, and an exciting frequency, corrects the wavelength of the selected light, which is obtained from the light to be measured.