1. A network design apparatus comprising: a processor configured: to perform a first process including: determining, in response to a plurality of demands each requesting traffic between a pair of nodes in a network in which one or more wavelength multiplexed optical signals are transmitted, a plurality of communication routes each providing a connection between the pair of nodes in the network, and estimating, for each link in the network, communication channels to be established in each communication route, in accordance with a wavelength constraint condition that a number of the communication channels to be established be smaller than or equal to a first upper limit for a number of available optical signal wavelengths, to perform a second process including assigning, for each link in the network, wavelengths of optical signals to be multiplexed into the one or more wavelength multiplexed optical signals, to the communication channels, based on a result of the estimating of the first process and in accordance with a system constraint condition that a number of optical signals that have an identical wavelength and are redundantly usable be smaller than or equal to a second upper limit for a number of available transmission systems each configured to transmit a wavelength multiplexed optical signal, and to perform a third process including: determining, for each link in the network, a target number of wavelengths for reducing a required number of transmission systems that is required for accommodating the plurality of demands, based on a result of the estimating of the first process and a result of the assigning of the second process, and determining whether there is a possibility of reducing the required number of transmission systems to be operated, by determining whether the target number of wavelengths is achievable by at least one of a change in bandwidths of the communication channels and a change in the plurality of communication routes; and a memory configured to store information on the network and the plurality of demands, wherein, upon determining that there is a possibility of reducing the required number of transmission systems, the third process changes the wavelength constraint condition so that the first upper limit is defined as the target number of wavelengths and changes the system constraint condition so that the second upper limit is reduced, with respect to the each link; the first process re-estimates the communication channels in accordance with the changed wavelength constraint condition; and the second process re-assigns wavelengths of the optical signals to be multiplexed, based on a result of the re-estimating of the first process and in accordance with the changed system constraint condition.