A clean gas circulates to pass through a loading area provided below a vertical heat treatment furnace. The clean gas unidirectionally flows through the loading area. After completion of wafer processing, a wafer boat lowers from the heat treatment furnace to the loading area, where the wafers are removed from the wafer boat. Subsequently, a clean gas jetting nozzle arranged in the loading area jets a clean gas toward the emptied wafer boat. Fragment of thin film which may readily peel off are blown away from the wafer boat, and are discharged out of the loading area together with the unidirectional flow. Thus, it is possible to avoid wafer contamination due to the unexpected peel-off of thin film fragments from the wafer boat.