The present invention is directed to a bidirectional add/drop multiplexer connected with waveguides at opposite ends thereof, respectively, and adapted to add/drop a channel in forward and reverse directions. The multiplexer includes a forward arrayed waveguide grating for demultiplexing the respective channels of the forward optical signal inputted thereto, for adding or dropping a channel to the demultiplexed channels of the forward optical signal, and for multiplexing the channel-added or dropped channels; a reverse arrayed waveguide grating for demultiplexing respective channels of the reverse optical signal inputted thereto, for adding or dropping a channel to the demultiplexed channels of the reverse optical signal, and for multiplexing the channel-added or dropped channels; a first circulator for inputting the forward optical signal to the forward waveguide grating and outputting the reverse optical signal received from the reverse waveguide grating to one of the waveguides, and, a second circulator for inputting the reverse optical signal to the forward waveguide grating and outputting the forward optical signal received from the forward waveguide grating to the other waveguide. Using this arrangement, the bidirectional add/drop multiplexer is capable of cutting off the relative intensity noise while avoiding the collision of the forward and reverse optical signals.