The Gas Slit Camera (GSC) is an X-ray instrument on the MAXI (Monitor of All-sky X-ray Image) mission on the International Space Station. It is designed to scan the entire sky every 92-minute orbital period in the 2–30 keV band and to achieve the highest sensitivity among the X-ray all-sky monitors ever flown so far. The GSC employs large-area position-sensitive proportional counters with the total detector area of 5350 cm$^2$. The on-board data processor has functions to format telemetry data as well as to control the high voltage of the proportional counters to protect them from the particle irradiation. The paper describes the instruments, on-board data processing, telemetry data formats, and performance specifications expected from the ground calibration tests.