PURPOSE: To monitor the setting state of an optical path by superimposing a 1st pilot tone to identify a wavelength of a signal light and a 2nd pilot tone to identify a path of a signal light onto the signal light, demultiplexing each pilot tone from the wavelength multiplex signal and analyzing the pilot tone.

CONSTITUTION: A wavelength multiplexed signal light is sent through optical fibers 11j (1≤j≤N) from different N locations. Let number of wavelength multiplexity be M, then the light intensity of the signal light by the M-waves is intensified altogether by a preamplifier 12j and the light is demultiplexed (13) into the signal light of each wavelength. The signal light of each wavelength is identified and recovered (14ij (1≤i≤M)) and the signal light of each wavelength to be recovered is switched for each optical path in an optical spatial switch 15j, M×N sets of optical paths in the optical path cross connect system are switched, M-kinds of PLTs to identify the wavelength of the signal and N-kinds of PLTs to identify the path of the signal are set and superimposed on the signal light. Thus, the setting state of the optical path is easily monitored.