An optical network comprising: a plurality of nodes which are interconnected via a transmission line; and a network management system for centrally managing operations of the plurality of nodes, wherein each of two or more nodes among the plurality of nodes comprises: an optical add-drop multiplexer configured to receive an optical signal transmitted through the transmission line, and configured to add or drop an optical signal having a predetermined wavelength to or from the optical signal; an optical receiver configured to receive an optical signal that is dropped at the optical add-drop multiplexer; an attenuator (333,337) configured to variably attenuate an optical signal power of each wavelength that is added from this node to the transmission line, on a wavelength-to-wavelength basis; an attenuation amount controller configured to control an attenuation amount corresponding to each wavelength at the attenuator; a monitor configured to monitor a quality of the optical signal that is received by the optical receiver, and configured to transfer receipt quality information that represents the monitoring result to the network management system; and an error correction controller configured to control a drive state of an error correction circuit for performing error correction on a received signal at the optical receiver, wherein the network management system comprises: an input unit through which optical network information is input; a threshold value setting unit configured to set two or more threshold values for an optical signal quality index which indicates a signal quality at a terminal node for an optical signal of each wavelength that is transmitted over the optical network, based on the optical network information that is input through the input unit; a computing unit configured to determine the optical signal quality index corresponding to each wavelength based on the receipt quality information transferred from the respective nodes, and configured to compute an attenuation amount corresponding to each wavelength at the attenuator in the respective nodes so that all of the optical signal quality indexes are equal to or greater than a lowest threshold value that is set at the threshold value setting unit and so that the number of wavelengths, which makes the optical signal quality index to be greater than the threshold values other than the lowest threshold value set at the threshold setting unit, is equal to or more than a target value; and a communication unit configured to transfer attenuation amount setting information, which indicates the computation result by the computing unit, to the attenuation amount controller, and configured to transfer error correction setting information, which indicates error correction setting corresponding to each wavelength that is determined based on the computation result by the computing unit, to the error correction controller. an optical add-drop multiplexer configured to receive an optical signal transmitted through the transmission line, and configured to add or drop an optical signal having a predetermined wavelength to or from the optical signal; an optical receiver configured to receive an optical signal that is dropped at the optical add-drop multiplexer; an attenuator (333,337) configured to variably attenuate an optical signal power of each wavelength that is added from this node to the transmission line, on a wavelength-to-wavelength basis; an attenuation amount controller configured to control an attenuation amount corresponding to each wavelength at the attenuator; a monitor configured to monitor a quality of the optical signal that is received by the optical receiver, and configured to transfer receipt quality information that represents the monitoring result to the network management system; and an error correction controller configured to control a drive state of an error correction circuit for performing error correction on a received signal at the optical receiver, an input unit through which optical network information is input; a threshold value setting unit configured to set two or more threshold values for an optical signal quality index which indicates a signal quality at a terminal node for an optical signal of each wavelength that is transmitted over the optical network, based on the optical network information that is input through the input unit; a computing unit configured to determine the optical signal quality index corresponding to each wavelength based on the receipt quality information transferred from the respective nodes, and configured to compute an attenuation amount corresponding to each wavelength at the attenuator in the respective nodes so that all of the optical signal quality indexes are equal to or greater than a lowest threshold value that is set at the threshold value setting unit and so that the number of wavelengths, which makes the optical signal quality index to be greater than the threshold values other than the lowest threshold value set at the threshold setting unit, is equal to or more than a target value; and a communication unit configured to transfer attenuation amount setting information, which indicates the computation result by the computing unit, to the attenuation amount controller, and configured to transfer error correction setting information, which indicates error correction setting corresponding to each wavelength that is determined based on the computation result by the computing unit, to the error correction controller.