A heat treatment apparatus includes a reaction tube extending in a first direction; a substrate support unit which is placed in the reaction tube and is configured to be capable of supporting plural substrates along the first direction; plural gas supply pipes provided at a side surface of the reaction tube to be aligned in the first direction with intervals for supplying a gas into the reaction tube; a gas dispersing plate which is provided in the reaction tube between opening edges of the plural gas supply pipes and the substrate support unit placed in the reaction tube, the gas dispersing plate being provided with plural opening portions formed to correspond to the gas supply pipes, respectively; and a heater which is placed outside the reaction tube for heating the substrates.