A substrate processing method is required in a process for forming a thin film on a substrate so as to improve the process efficiency and form a high quality thin film. To this end, the substrate processing method according to the present invention comprises: forming an n-type layer including a group-III element and a group-V element on a substrate in the first chamber by a vapor deposition process; carrying the substrate out of the first chamber to a buffer chamber then into the second chamber different from the first chamber; and forming an active layer including a group-III element and a group-V element on a substrate in the second chamber by a vapor deposition process.