The present invention relates to a solid-state image pickup element capable of transmitting pixel signals read from a pixel unit as optical signals at high speed, an optical apparatus, a signal processing apparatus, and a signal processing system.The solid-state image pickup element 1A includes a pixel unit 10A which converts light into electric signals, an A/D convertor 11A which converts the signals read from the pixel unit 10A into digital signals, an optical communication unit 12A which converts the signals digitalized by the A/D convertor 11A into optical signals and outputs the optical signals, a timing generator 13A which generates a driving clock used to synchronize processes of inputting/outputting signals performed by the pixel unit 10A, the A/D convertor 11A, and the optical communication unit 12A, and a controller 16A which controls reading of signals.