According to an embodiment of the present invention, a substrate processing apparatus includes: a chamber in which a process for a substrate is performed; a showerhead installed in the chamber to inject a reaction gas toward the substrate; and a susceptor installed below the showerhead to support the substrate. Here, the showerhead includes: a showerhead main body including an inner space to which the reaction gas is supplied from the outside and a plurality of injection holes configured to inject the reaction gas while communicating with the inner space; an inflow plate installed in the inner space to divide the inner space into an inflow space and a buffer space and including a plurality of inflow holes configured to allow the inflow space and the buffer space to communicate with each other; and a plurality of adjustment plates installed on the inflow holes in a movable manner, respectively, and configured to restrict movement of the reaction gas from the inflow space to the buffer space.