An optical circuit includes a selector that supplies an output current from a first photodetector to a converter provided for a second photodetector, and a processor that imaginarily calculates the level of signal light output from an optical device, which is detected by the second photodetector, based upon the level of signal light input to the optical device, which is measured by measurement means, and upon conversion efficiencies of the first and second photodetectors and that thus calculates the correlation between the output signal light level and its monitored value output from the converter provided for the second photodetector.