A gas injection apparatus includes a supply block including central gas ports configured to supply a process gas provided from a gas supply source; a diffusion cover under the supply block and including a branch duct which is connected to the central gas ports; a plurality of spray nozzles on a lower surface of the diffusion cover and connected to the branch duct, the plurality of spray nozzles configured to spray the process gas; and a shower head under the plurality of spray nozzles and including a plurality of spray holes vertically passing through the shower head, the plurality of spray holes configured to evenly spray the process gas sprayed by the plurality of spray nozzles. The diffusion cover further includes a concave portion at an upper surface thereof, the concave portion facing the central gas ports and concaved downward.