In a substrate processing apparatus for processing a substrate mounted on a mounting table in a processing chamber by supplying a gas to the substrate, the apparatus includes: a partition unit provided, between a processing space where a substrate is provided and a diffusion space where a first gas is diffused, to face the mounting table; a first gas supply unit for supplying the first gas to the diffusion space; first gas injection holes, formed through the partition unit, for injecting the first gas diffused in the diffusion space into the processing space; and a second gas supply unit including second gas injection holes opened on a gas injection surface of the partition unit which faces the processing space. The second gas supply unit independently supplies a second gas to each of a plurality of regions arranged in a horizontal direction in the processing space separately from the first gas.