A deposition apparatus configured to form a thin film on a substrate includes: a reactor wall; a substrate support positioned under the reactor wall; and a showerhead plate positioned above the substrate support. The showerhead plate defines a reaction space together with the substrate support. The apparatus also includes one or more gas conduits configured to open to a periphery of the reaction space at least while an inert gas is supplied therethrough. The one or more gas conduits are configured to supply the inert gas inwardly toward the periphery of the substrate support around the reaction space. This configuration prevents reactant gases from flowing between a substrate and the substrate support during a deposition process, thereby preventing deposition of an undesired thin film and impurity particles on the back side of the substrate.