Abstract: An optical communication system using a four-wavelength add/drop type wavelength division multiplexer (WDM) is provided for extending a communication coverage without adding an optical line and a repeater by installing a wavelength-pass add/drop type optical module at an optical line connected between an existing transmitting station or base station and an existing receiving station or repeater. The optical communication system includes a slave unit including, a first WDM for dividing or mixing a first wavelength and a second wavelength, a second WDM which is connected to the first WDM and divides or mixes the first wavelength and the second wavelength to allow them travel through a predetermined path, a third WDM which is connected to the first WDM and divides a third wavelength and a fourth wavelength from a communication line of the second wavelength; and a fourth WDM which is connected to the second WDM and multiplexes or demultiplexes the third wavelength and the fourth wavelength.