An optical transmission line switching apparatus adapted to carry out system switching between an optical transmission line of a single active system and an optical transmission line of at least one backup system, the optical transmission line switching apparatus comprising: - input optical power monitors (10a, 10b) that are provided to the optical transmission lines, respectively, adapted to monitor input optical powers of signal light rays input from the optical transmission lines; - first variable optical attenuators (13a, 13b) that are placed downstream of the input optical power monitors (10a, 10b), respectively, adapted to attenuate and output the signal light rays input from the optical transmission lines; - a multiplexer adapted to multiplex the signal light rays output from the first variable optical attenuators (13a, 13b); - a threshold setter adapted to set a threshold for the input optical power monitor to detect the input optical power to the optical transmission line of the active system; - a threshold decider adapted to decide 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 setter or not; and - an attenuation controller adapted to carry out, when the threshold decider decides that the input optical power is not greater than the threshold, system switching by controlling the first variable optical attenuators (13a, 13b) so as to gradually reduce attenuation of the signal light rays input from the optical transmission line of the at least one backup system, and to gradually increase attenuation of the signal light rays input from the optical transmission line of the active system. - input optical power monitors (10a, 10b) that are provided to the optical transmission lines, respectively, adapted to monitor input optical powers of signal light rays input from the optical transmission lines; - first variable optical attenuators (13a, 13b) that are placed downstream of the input optical power monitors (10a, 10b), respectively, adapted to attenuate and output the signal light rays input from the optical transmission lines; - a multiplexer adapted to multiplex the signal light rays output from the first variable optical attenuators (13a, 13b); - a threshold setter adapted to set a threshold for the input optical power monitor to detect the input optical power to the optical transmission line of the active system; - a threshold decider adapted to decide 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 setter or not; and - an attenuation controller adapted to carry out, when the threshold decider decides that the input optical power is not greater than the threshold, system switching by controlling the first variable optical attenuators (13a, 13b) so as to gradually reduce attenuation of the signal light rays input from the optical transmission line of the at least one backup system, and to gradually increase attenuation of the signal light rays input from the optical transmission line of the active system.