In a rapid thermal processing (RTP) of a large-diameter wafer, a wafer is heat treated by an upper high-temperature furnace and a lower low-temperature furnace, which are separated from and can be brought into close contact with one another by a relative vertical position adjusting means. The upper high-temperature furnace has an open bottom which is shut by an openable, heat insulating shutter. Height of the apparatus as a whole can be shortened.