Disclosed is a WDM system for demultiplexing mixed optical signals transmitted through one channel from the outside to distribute specific wavelength optical signals into a plurality of channels. The WDM system has a receiving optical fiber for receiving the mixed optical signals. A filter is arranged in an output end of the receiving optical fiber for selectively transmitting a specific optical signal of a wavelength identical with the peak wavelength of the filter but reflecting remaining wavelength optical signals. A transmitting optical fiber outputs the specific wavelength optical signal transmitted through the filter. A shutter member attenuates the specific wavelength optical signal between the filter and the transmitting optical fiber. An actuator drives the shutter member across the propagation of the specific wavelength optical signal transmitted through the filter; and a control unit for controlling the actuation of the actuator. The built-in optical attenuators allow integration of individual components, simplify the structure of the system as well as improve the performance and reliability of the system.