An optical signal processing apparatus includes a removal unit and a superimposition unit. The removal unit receives, from a first optical transmission path, an optical signal converted from an electrical signal, in which a first signal and a second signal having different frequencies from each other have been superimposed, and removes the second signal from the optical signal which has been input. The superimposition unit superimposes a third signal having a frequency different from a frequency of the first signal on the optical signal in which the second signal has been removed by the removal unit, and outputs the optical signal in which the third signal has been superimposed to a second optical transmission path.