A heat treatment apparatus includes: a vertically-extended reaction tube for accommodating a substrate holder with substrates are vertically stacked therein; a gas supply duct integrally formed with the reaction tube to extend in a longitudinal direction of the reaction tube; gas supply holes formed in a region of an outer circumferential wall of the reaction tube and configured to bring an interior of the gas supply duct and an interior of the reaction tube into communication with each other; a preheating duct communicating with the gas supply duct, integrally formed with the reaction tube to extend along the longitudinal direction of the reaction tube up to a predetermined position of a predetermined height existing in the vicinity of an upper end of the outer circumferential wall of the reaction tube; and a pressure-resistant container for covering the reaction tube, the gas supply duct and the preheating duct.