PURPOSE: To decrease a DA conversion processing time by utilizing the wavelength dependency of a light absorbing coefficient of an active layer to convert plural pulse light beams simultaneously into plural analog light beam stimulating currents.

CONSTITUTION: When a digital optical signal is made incident in an active layer 3, a light stimulating current of 130μA flows to an optical modulator, which converts the current into an analog electric signal. In such a case, since 4 optical signals are simultaneously made incident, the digital optical signal is decoded into an analog electric signal in 1-bit time and the conversion speed goes to 4 times.