Abstract: A heat treating device including a processing vessel for exposing objects to be treated (e.g., semiconductor wafers) held in a wafer boat for a heat treatment, processing gas introduction nozzles for introducing processing gases, a cap for openably closing tightly an opening of the processing vessel; a heat retention cylinder on which the boat is mounted; and a gas introduction opening, a gas passage, and a gas release opening opened in the heat retention cylinder. Processing gases introduced from the introduction nozzles are taken into the heat retention cylinder through the gas introduction opening and released radially from the gas release opening in the top thereof. The heat retention cylinder has the function of introducing the processing gases, whereby high intra-plane film thickness uniformity can be retained without rotating the wafer boat.