An optical transmission system for removing skew between optical channels is provided. The optical transmission system comprises an optical signal transmitter and an optical signal receiver. The optical transmitter inserts serial numbers into an overhead of a frame, delays the frame by a time corresponding to skew information transmitted from an opposite optical transmission system, and transmits the frame. The optical signal receiver receives the frame from the opposite optical transmission system via fiber optics, removes skew existing in the received frame in units of both a byte and a frame by referring to the serial numbers stored in the overhead of the frame, and transmits the skew information collected when the skew is removed, to the opposite optical transmission system.