An apparatus for processing a substrate includes a load port on that a substrate transfer container accommodating a substrate is laid, a processing part configured to process a substrate, and an index part including an index robot configured to carry a substrate between the substrate transfer container and the processing part, wherein the processing part includes a first transfer chamber configured to have a first main carrying robot carrying a substrate and disposed adjacent to the index part, a second transfer chamber configured to have a second main carrying robot carrying a substrate and disposed adjacent to the first transfer chamber, and a shuttle buffer part configured to move between the first transfer chamber and the second transfer chamber for transferring a substrate between the second main carrying robot and the index robot.