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.