<p>An optical spectrum analyzer is formed by providing an optical waveguide, a SAW waveguide for propagating an acoustic surface wave to convert the mode of light propagated through the optical waveguide, and an IDT for exciting the acoustic surface wave propagated through the SAW waveguide on a substrate and includes an acousto-optic tunable filter (AOTF) which light from a broadband light source enters and which causes emitted light to enter a device under test (DUT). The optical spectrum analyzer is provided with the device under test (DUT) between the acousto-optic tunable filter (AOTF) and a light receiver and obtains the optical spectrum of the device under test (DUT) from the relation between a high-frequency signal of the acousto-optic tunable filter (AOTF) to be sent to the IDT and a light-receiving signal intensity of the light receiver. The low-cost optical spectrum analyzer for measuring the spectrum of transmitted light or reflected light of the device under test can be obtained by this constitution. Multichannel measurement is possible without reducing the measuring speed.</p>
