Abstract: Provided are a gas injection device and substrate processing apparatus using the same. The gas injection device includes a plurality of gas injection units disposed above a substrate support part rotatably disposed within a chamber to support a plurality of substrates, the plurality of gas injection units being disposed along a circumference direction with respect to a center point of the substrate support part to inject a process gas onto the substrates. Each of the plurality of gas injection units includes a top plate in which an inlet configured to introduce the process gas is provided and an injection plate disposed under the top plate to define a gas diffusion space between the injection plate and the top plate along a radius direction of the substrate support part, the injection plate having a plurality of gas injection holes under the gas diffusion space to inject the process gas introduced through the inlet and diffused in the gas diffusion space onto the substrate.