A gas supply device of supplying a gas into a processing space from a gas supply source includes a facing plate that faces the processing space and includes multiple through holes; multiple gas distribution plates; and a cover plate. The facing plate, the gas distribution plates, and the cover plate are stacked in sequence. In a surface, which faces the facing plate, of the gas distribution plate closest to the facing plate, multiple gas diffusion spaces including a first gas diffusion space and a second gas diffusion space are formed, and in each of the gas distribution plates, a first gas supply path through which a processing gas or an additional gas is supplied into the first gas diffusion space and a second gas supply path through which the processing gas or the additional gas is supplied into the second gas diffusion space are formed.