PURPOSE: To realize quick response, size reduction, and high reliability by providing an optical multiplexer demultiplexer consisting of waveguide optical switches which uses electro-optic effect on one substrate.

CONSTITUTION: Waveguide optical switches 81 and 82 which operate by electro- optic effect while connected in a tree shape are formed on the same substrate to obtain high-speed response and high reliability. Further, electrodes of each stage are connected together electrically to facilitate address control. Then, a control voltage for the waveguide switches to be supplied is obtained from a photoelectromotive force generated when photoelectromotive force elements 4W4" are irradiated with control light beams 7W7" to make the device tolerant to an electromagnetically induced noise, and the control light beams are emitted on wavelength multiplex basis to obtain the multiplexer demultiplexer which decreases transmission lines for the control light beams.