Abstract: Wavelength multiplex terminal apparatuses (2 and 5) comprise transmissionreception devices (21 to 2N or 51 to 5N) in which light signals of 10-bit bit strings transmitted by transmission devices (11 to 1N or 61 to 6N) are converted into codes of 8-bit bit strings, according to a certain encoding rule, and presence of any code errors and RD errors are detected. The apparatuses (2 and 5) output transmission quality monitor signals and transmit them to a monitor and control device 8 via monitoring devices (2c or 5c) and a network (7) such that transmission qualities of the signals of the apparatuses (2 and 5) within a wavelength multiplex interval can be monitored.