In order to measure the signal quality of each of optical signals transmitted/received via a plurality of transmission lines, an optical communication system 1 is provided with a dummy light source 10 for outputting dummy light, a switching means 20 for outputting the dummy light to a first transmission line 40a, and a light-receiving means 30 for acquiring first signal quality from the dummy light received via the first transmission line 40a, the switching means 20 switching the output destination of the dummy light from the first transmission line 40a to a second transmission line 40b, and the light-receiving means 30 acquiring second signal quality from the dummy light received via the second transmission line 40b.