A network system comprising: a control device; and an add/drop multiplexer, wherein the control device comprising: an interface configured to be capable of communicating with the add/drop multiplexer; and a first controller capable of controlling the add/drop multiplexer via the interface, wherein the first controller is adapted to transmit, to the add/drop multiplexer, the connection information indicating that a device connected to the add/drop multiplexer is changed, when the device connected to the add/drop multiplexer is changed, the add/drop multiplexer comprising: a transmitter configured to be capable of selectively transferring a received wavelength multiplex optical signal to at least one client device and at least one network for each wavelength constituting the wavelength multiplex optical signal; and a second controller configured to indicate a transfer destination of an optical signal at a wavelength according to an attribute of the optical signal at the wavelength to the transmitter, the wavelength being included in the wavelength multiplex optical signal, wherein the second controller is adapted to indicate the attenuation amount specified for each device connected to the transmitter to the transmitter, when the second controller receives the connection information indicating that the device connected to the add/drop multiplexer is changed the transmitter is adapted to attenuate the optical signal at the wavelength according to the attenuation amount and transfer the attenuated optical signal to the transfer destination. a control device; and an add/drop multiplexer, wherein the control device comprising: an interface configured to be capable of communicating with the add/drop multiplexer; and a first controller capable of controlling the add/drop multiplexer via the interface, wherein the first controller is adapted to transmit, to the add/drop multiplexer, the connection information indicating that a device connected to the add/drop multiplexer is changed, when the device connected to the add/drop multiplexer is changed, an interface configured to be capable of communicating with the add/drop multiplexer; and a first controller capable of controlling the add/drop multiplexer via the interface, wherein the first controller is adapted to transmit, to the add/drop multiplexer, the connection information indicating that a device connected to the add/drop multiplexer is changed, when the device connected to the add/drop multiplexer is changed, the add/drop multiplexer comprising: a transmitter configured to be capable of selectively transferring a received wavelength multiplex optical signal to at least one client device and at least one network for each wavelength constituting the wavelength multiplex optical signal; and a second controller configured to indicate a transfer destination of an optical signal at a wavelength according to an attribute of the optical signal at the wavelength to the transmitter, the wavelength being included in the wavelength multiplex optical signal, wherein the second controller is adapted to indicate the attenuation amount specified for each device connected to the transmitter to the transmitter, when the second controller receives the connection information indicating that the device connected to the add/drop multiplexer is changed the transmitter is adapted to attenuate the optical signal at the wavelength according to the attenuation amount and transfer the attenuated optical signal to the transfer destination. a transmitter configured to be capable of selectively transferring a received wavelength multiplex optical signal to at least one client device and at least one network for each wavelength constituting the wavelength multiplex optical signal; and a second controller configured to indicate a transfer destination of an optical signal at a wavelength according to an attribute of the optical signal at the wavelength to the transmitter, the wavelength being included in the wavelength multiplex optical signal, wherein the second controller is adapted to indicate the attenuation amount specified for each device connected to the transmitter to the transmitter, when the second controller receives the connection information indicating that the device connected to the add/drop multiplexer is changed the transmitter is adapted to attenuate the optical signal at the wavelength according to the attenuation amount and transfer the attenuated optical signal to the transfer destination.