An optical signal control device (70) is configured to include: a leakage amount calculating unit (83) calculating, from a light intensity measured by a first light intensity measuring unit (77) and a light intensity measured by a second light intensity measuring unit (78), a leakage amount of light leaking from other optical signals to each of optical signals included in a combined signal; and an attenuation amount calculating unit (84) calculating, from the light intensity measured by the second light intensity measuring unit (78) and the leakage mount of light, an attenuation amount of each of the optical signals included in the combined signal, and a wavelength selective switch (71) attenuates each of the optical signals included in the combined signal depending on the attenuation amount calculated by the attenuation amount calculating unit (84).