An optical receiver receives signal light obtained by superimposing a second signal to a first signal according to a predetermined superimposition pattern. In the optical receiver, a tunable optical filter allows a part of a spectrum of the received light to pass, and a photodetector converts the transmission light into an electrical signal. A filter controller controls a center transmittance frequency of the tunable optical filter in response to a superimposed period and a non-superimposed period of the second signal identified based on the electrical signal. A superimposed signal detector detects the second signal based on the electrical signal obtained in response to the control of the tunable optical filter during the superimposed period. An optical signal-to-noise ratio (OSNR) calculator calculates an OSNR of the signal light based on the electrical signal obtained in response to the control of the tunable optical filter during the non-superimposed period.