Abstract: A heat-treating apparatus for heat treating a plurality of objects-to-be-treated (semiconductor wafers) by loading the objects-to-be-treated into a heat treatment vessel having the lower end opened, mounted on a heat-treatment boat vertically spaced from each other, and sealing the opened end includes a heat insulator disposed on the lower end portion of the heat-treatment boat for heat-insulating the interior of the reaction vessel. The heat-insulator includes heat transmission preventing plates of opaque quartz for preventing the transmission of heat rays from a heater, and a support for supporting the heat transmission preventing plates. Thus sufficient heat-insulating effect for a heat-treating operation can be achieved, and stable heat-treatment can be conducted without impairing the function of sealing the heat-insulating unit. Furthermore, particle formation can be prevented, with a result of improved yields of the heat-treatment.