Abstract: The present invention relates to a heat treatment apparatus wherein treatment objects such as semiconductor wafers contained in a treatment boat are loaded in a treatment container such as a process tube. Water vapor is supplied from the top of the treatment container toward the bottom for heat treatment of the treatment objects to permit water vapor passage between the treatment container top and the top face of the heat treatment boat. A gas diffusion plate possessing for example 16 flow holes is provided, moreover, a heat treatment space is formed at the bottom direction of the gas diffusion plate. These flow holes are arranged at equal intervals in the circumference direction of a space between the outer circumference of the treatment objects held by the heat treatment boat and the inner side of the treatment container.