  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.
