A technique capable of preventing by-products from adhering to a lower portion of a process vessel utilizes a substrate processing apparatus including: a process vessel having a process chamber; a lid configured to close a lower end opening of the process vessel; a substrate retainer; an insulating structure; a process gas supply mechanism configured to supply a process gas; a purge gas supply unit configured to supply a purge gas to a lower region of the process vessel via a gap between the insulating structure and the lid; and a restrictor disposed in the gap. The restrictor regulates flow of the purge gas such that the flow rate of the purge gas supplied to a first portion of the lower region of the process vessel is greater than a flow rate of the purge gas supplied to a second portion of the lower region of the process vessel.