A solid-state image pickup device includes a pixel unit configured to convert light into an electrical signal, an A/D converter configured to convert a signal read from the pixel unit into a digital signal, a light modulation unit configured to modulate an externally input light beam using the signal digitized by the A/D converter and output a signal light beam based on the signal read from the pixel unit, a timing generation unit configured to generate a synchronization signal used for synchronizing input and output of signals of the pixel unit, the A/D converter, and the light modulation unit, and a controller configured to control readout of the signal.