A solid-state imager includes a pixel unit for converting incident light into an electrical signal, a substrate bearing the pixel unit formed thereon, an analog-to-digital converter, formed on the substrate, for converting a signal read from the pixel unit into a digital signal, an optical communication unit, arranged on the bottom surface of the substrate opposite the surface of the substrate bearing opposite the top surface of the substrate bearing the pixel unit receiving the incident light, for converting the digital signal converted by the analog-to-digital converter into a light signal and outputting the light signal, and a signal line for transferring the digital signal, converted by the analog-to-digital converter, to the optical communication unit arranged on the bottom surface of the substrate.