A gas supply system according to the present disclosure includes a first flow channel connected to a first gas source of a first gas, formed inside a ceiling or a sidewall of the treatment container, and communicating with the treatment space through a plurality of first gas discharge holes. The gas supply system also includes a second flow channel connected to a second gas source of a second gas, formed inside the ceiling or the sidewall of the treatment container, and communicating with the treatment space through a plurality of second gas discharge holes. The gas supply system further includes a plurality of first diaphragm valves, wherein each of the first diaphragm valves is provided between the first flow channel and the first gas discharge hole to correspond to the first gas discharge hole.