Disclosed is a substrate processing apparatus including: a reaction tube to accommodate at least one substrate; at least a pair of electrodes disposed outside the reaction tube; and a dielectric member, wherein a plasma generation region is formed at least in a space between an inner wall of the reaction tube and an outer circumferential edge of the substrate, the member includes a main face extending in a radial direction of the substrate and in a substantially entire circumferential direction of the substrate in a horizontal plane parallel to a main face of the substrate, and is disposed in an outer circumferential region of the substrate, and gas activated in the plasma generation region is supplied through a surface region of the main face of the member to the substrate.