A gas supply device disposed opposite to a substrate mounted on a loading board in a processing container and supplying a process gas for processing the substrate comprises a top plate member having a recess formed to spread gradually toward the state in order to constitute a gas diffusion space at a position facing the substrate on the loading board, and a gas supply nozzle projecting into the recess from the top thereof and having a plurality of gas supply holes along the circumferential direction of the recess.