A method of determining a transmission route for optical signals in an optical signal transmission system comprising a transmitter for optical signals, a receiver for the optical signals, and repeaters for repeating the optical signals transmitted from the transmitter to the receiver, and having transmission routes between the transmitter and the receiver, wherein the numbers of the wavelength components of the optical signals that has been in-use in each section defined to each of the transmission routes, the acquired numbers of wavelength components are compared among the transmission routes, and one of the transmission routes having the minimum number of wavelength components is determined as the transmission route used to set the line.