An apparatus includes: a photodetector configured to create a first electric-signal from an optical signal; a power-measuring unit configured to measure power of the optical signal according to the first electric-signal; a noise calculating unit configured to calculate noise corresponding to a specified target optical signal-to-noise ratio (OSNR) according to the power of the optical signal, the power having been measured by the power-measuring unit, the specified target optical signal-to-noise ratio, and information representing characteristics of the photodetector; a noise generating unit configured to add the noise calculated by the noise calculating unit to the first electric-signal to generate a second electric-signal; an OSNR measuring unit configured to measure an optical signal-to-noise ratio according to the second electric-signal; and a calibration coefficient calculating unit configured to calculate a calibration coefficient used to obtain the target optical signal-to-noise ratio from the optical s
