Disclosed is an interleaver for optical communication system. A dual collimator is combined with first optical fiber acting as a light input port and one end of second optical fiber spaced from the first optical fiber, and switches light advancing along the first and second optical fibers into parallel light. A beam splitter is arranged so that one surface is opposite to the dual collimator, and transmits one half of an input light and reflects the other half of the input light. A single collimator is arranged opposite to the other surface of the beam splitter, and combined with the other end of the second optical fiber so as to switch light advancing toward the other end of the second optical fiber into parallel light. An output optical fiber is arranged opposite to the other surface of the beam splitter and acts as a light output port.