A network design method includes calculating, from the number of wavelength filters on a transmission route of an optical signal, a bandwidth of the optical signal after narrowing by the wavelength filters, selecting, from a plurality of combinations of a multi-level modulation system and a baud rate about the optical signal set in a transmitting apparatus, one or more first combinations about which a lower-limit value of the bandwidth of the optical signal in each of the combinations is equal to or smaller than the bandwidth of the optical signal after narrowing, and selecting a second combination from the one or more first combinations based on a minimum value of an OSRN of the optical signal in the selected combinations.