An optical receiver including a variable optical attenuator for controlling an optical loss value for each optical transmission line based on a predetermined optical loss value; a variable dispersion compensation module for controlling a wavelength dispersion value for each optical transmission line based on a predetermined dispersion value; an optical loss/dispersion controller for measuring the optical loss value and the wavelength dispersion value of every optical transmission line, outputting these values so that they are equal in all the optical transmission lines, and controlling the attenuator and the module based on the outputted predetermined values; a receive amplifier for receiving the optical signal whose light level is kept constant and amplifying the signal; and, a transmission line switch control module is provided for switching a working transmission line into a protection line if the optical signal level of the working line is lower than a threshold value.