In an embodiment of the invention, an optical transport apparatus comprises first wavelength-division-multiplex optical signal processing units each corresponding to each of transmission lines, an interface unit connected to a client apparatus and converting a client signal to and from a signal to be wavelength-division multiplexed, and a second wavelength-division-multiplex optical signal processing unit connected to the interface unit and receiving the signal to be wavelength-division multiplexed from the interface unit. Each of the first wavelength-division-multiplex optical signal processing units includes a wavelength selective switch including inputs, multiplexing signals selected from optical input signals received from the inputs and outputting the multiplex signal to a corresponding transmission line. The second wavelength-division-multiplex optical signal processing unit includes an optical splitter splitting an optical signal including the signal to be wavelength-division multiplexed and outputting the split signals to the wavelength selective switches in the first wavelength-division-multiplex optical signal processing units.