PURPOSE: To convert a light signal from serial to parallel as it is the light signal without using any roundabout means by providing one waveguide on a substrate with a means which controls coefficients of optical coupling at coupling parts of plural light guides coupled at bit intervals of light.

CONSTITUTION: N coupling parts 4-1W4-N which couple a waveguide 2 with waveguides 3-1W3-N are arranged at bit intervals VT (V: group speed of light signal in waveguide 2, T: bit period). A voltage with pulse width T and a repetitive period NT is applied to electrodes arranged at the coupling parts 4-1W4-N at the same time and in synchronization with the light digital signal in the waveguide 2, so that the light signal is branched to the waveguides 3-1W3-N by N bits each in parallel. On the other hand, N-bit input parallel light signals from the N waveguides 3-1W3-N are coupled with the waveguide 2 through the coupling parts 4-1W4-N only when a voltage with a proper phase is applied in synchronization with the light signals.