A communication system in which a master station device and a plurality of slave station devices (20-1,...,20-n) are connected by an optical transmission line, and at least one of upstream communication and downstream communication is carried out using a plurality of wavelengths, wherein the master station device comprises: a control unit that is configured to assign the slave station device a wavelength to be used in the communication, and to generate a first control signal for notifying the slave station device of the assigned wavelength; and an optical transmitter is configured to transmit the first control signal generated by the control unit to the slave station device, and wherein the slave station device comprises: an optical transceiver (23, 24) is configured to carry out communication with the master station device using the wavelength based on the first control signal received from the master station device, wherein the optical transceiver of the slave station device is configured to transmit to the master station device a first information on a switching time needed to switch a wavelength used by the slave station device; characterized in that when the wavelength used by the slave station device is switched, the control unit of the master station device is configured to estimate, based on the first information, a switching completion time of the slave station device, and the optical transmitter of the master station device is configured to transmit based on the estimated switching completion time, a switching completion notification to the slave station device. a control unit that is configured to assign the slave station device a wavelength to be used in the communication, and to generate a first control signal for notifying the slave station device of the assigned wavelength; and an optical transmitter is configured to transmit the first control signal generated by the control unit to the slave station device, and wherein the slave station device comprises: an optical transceiver (23, 24) is configured to carry out communication with the master station device using the wavelength based on the first control signal received from the master station device, wherein the optical transceiver of the slave station device is configured to transmit to the master station device a first information on a switching time needed to switch a wavelength used by the slave station device; characterized in that when the wavelength used by the slave station device is switched, the control unit of the master station device is configured to estimate, based on the first information, a switching completion time of the slave station device, and the optical transmitter of the master station device is configured to transmit based on the estimated switching completion time, a switching completion notification to the slave station device. an optical transceiver (23, 24) is configured to carry out communication with the master station device using the wavelength based on the first control signal received from the master station device, wherein the optical transceiver of the slave station device is configured to transmit to the master station device a first information on a switching time needed to switch a wavelength used by the slave station device; characterized in that when the wavelength used by the slave station device is switched, the control unit of the master station device is configured to estimate, based on the first information, a switching completion time of the slave station device, and the optical transmitter of the master station device is configured to transmit based on the estimated switching completion time, a switching completion notification to the slave station device.