PURPOSE: To make the speed of conversion unrestricted by the speed of processing by forming plural light leaking parts at intervals corresponding to the distance advanced by light in the optical path in clock interval time of serial signals.

CONSTITUTION: Intervals of light leaking parts are so set that serial optical signals are arranged in a line in each signal reading section. When serial light signals are set in from an end 14 of optical fiber 1, the foremost part of the optical signals arrives at the foremost light leaking part through each light leaking part. At the same time, serial light signals are positioned successively at each light leaking part, and leak out at respective light leaking part and read out by signal reading sections 31∼34. In this case, a signal reading device 3 is so controlled that erroneous signal reading is not made even when signal light is leaking from other light leaking part until foremost light signal arrives at the foremost light leaking part. In the case where the signal reading device is CCD, the signal charge read out successively is transferred in due turn and outputted as electrical signals.