An optical add/drop multiplexer includes a multiplexing/demultiplexing unit having an input multiplexing port, an output multiplexing port and a plurality of demultiplexing ports each connected to a respective optical circulator. Each optical circulator includes a first port for internally inputting an added channel, a second port for outputting the added channel to a corresponding demultiplexing port and receiving a channel outputted by the corresponding demultiplexing port, and a third port for outputting the channel received from the second port. Each of optical switches is located between adjacent circulators, wherein each optical switch includes a first input port, coupled to the third port of one optical circulator, for receiving the channel from the third port, a second input port for receiving an externally added channel, a first output port, coupled to another first port of another optical circulator, for outputting the added channel to another first port, and a second output port for externally outputting the channel inputted through the first input port according to the dropping operation.