An optical packet switching system includes an optical packet generator for generating an optical packet signal, an optical packet switching unit, provided with an optical switch, for switching the route of an inputted optical packet signal by controlling on/off of the optical switch, and an optical signal-to-noise ratio measuring unit for measuring the optical signal-to-noise ratio of the optical packet signal outputted from the optical packet switching unit. When switching the route of the optical packet signal, the optical packet switching unit outputs an optical packet signal with optical noise by keeping the optical switch on longer than the time width of the packet signal. The optical signal-to-noise ratio measuring unit measures the optical signal power and the optical noise power, respectively, in the optical packet signal with optical noise and measures the optical signal-to-noise ratio by calculating the ratio between the optical signal power and the optical noise power.