A shower head 13 (gas supply device) of supplying a processing gas and an additional gas from a processing gas supply source and an additional gas source, respectively, into a processing space S includes multiple gas distribution plates 28 to 31, the cooling plate 32, and the cover plate 33 stacked in sequence. A peripheral gas diffusion space 35 and an outermost gas diffusion space 36 are formed in the undermost gas distribution plate 28. At least one gas supply path for supplying the processing gas and additional gas into any one of the peripheral gas diffusion space and the outermost gas diffusion space is formed at each of the gas distribution plates. The gas supply path is branched into multiple lines and distances from the processing gas supply source to front ends of the respective branch lines are set to be all the same.