A bi-directional optical add/drop multiplexer is provided comprising a first interleaver for outputting to a second end a forward optical signal that has been inputted through a first end connected to the optical fiber, and a backward optical signal to a second end that has been inputted through a third end; a second interleaver for interleaving the forward optical signal inputted through a first end and the backward optical signal inputted through a second end by wavelengths, and outputting the interleaved signal through a third end; an optical add/drop multiplexing unit for adding a selected channel to the interleaved optical signal or dropping a selected channel from the interleaved optical signal; a third interleaver for outputting a backward optical signal among the channel-modified optical signals inputted through a second end to the third end of the first interleaver through a first end, and a forward optical signal through a third end; and a fourth interleaver for outputting a backward optical signal inputted through a second end connected to an optical fiber to the second end of the second interleaver through a first end, and the forward optical signal inputted from the third interleaver to the second end.