A slave station device including: a superposition unit that is capable of superposing a main signal and a master station control signal by an electrical domain superposition method in which the main signal and the master station control signal for controlling a master station device connected to the slave station device are converted into an optical signal after multiplexing and an optical domain superposition method in which the main signal is modulated into an optical signal and then optically modulated by the master station control signal, and superposes the main signal and the master station control signal by any one of the electrical domain superposition method and the optical domain superposition method; and a control unit that controls the superposition method by the superposition unit on the basis of a slave station control signal indicating the superposition method received from the master station device.