PURPOSE: To convert light data inputted in parallel directly to serial light data without converting to electrical signals by inputting light data to be converted from opposite side to confluent end of plural light paths and outputting light data led to the confluent end.

CONSTITUTION: Optical fibers 1∼4 are wound specified number of times on a proper cylindrical support 6 and fixed by fixing means and an end of each fiber is joined at a point A by means of, for instance, welding etc. and connected to an end of an optical fiber 5. Number of times of winding of respective optical fiber on the support 6 is set in arithmetic progression. When parallel light data of 4 bits corresponding to an input terminal 11 in higher rank bit are inputted to input terminals 11∼14, the light data arrive at the confluent point A successively from data of higher rank bit delaying by a delay time. Thus, the data are outputted serially from an output terminal 51 through an optical fiber 5 and parallel serial conversion is achieved.