A device includes a threshold setting unit that sets a threshold for an input optical power monitor to detect the input optical power to the optical transmission line of an active system; a threshold deciding unit that decides whether the input optical power to the optical transmission line of the active system detected by the input optical power monitor is not greater than the threshold set by the threshold setting unit or not; and an attenuation controller that carries out, when the threshold deciding unit decides that the input optical power is not greater than the threshold, system switching by controlling first variable optical attenuators so as to gradually reduce attenuation of the signal light rays input from the optical transmission line of one backup system, and to gradually increase attenuation of the signal light rays input from the optical transmission line of the active system.
