Described herein is a technique capable of suppressing adhesion of by-products to a furnace opening portion. A substrate processing apparatus includes: a reaction vessel having an opening at a lower end and accommodating a substrate retainer; a shaft rotatably supporting the substrate retainer; a cap including: a side surface portion having a predetermined gap with an inner surface of the reaction vessel; a cylindrical portion through which the shaft is inserted; an upper plate portion of an annular shape; and a flange connected to a lower end of the side surface portion; and a cap cover connected to the shaft above the upper end of the cylindrical portion. A purge gas from thereunder flows sequentially to a space between the shaft and the cylindrical portion, a space between the upper plate portion and the cap cover and a space between the side surface portion and the cap cover.