A heat treatment apparatus for semiconductor wafers is provided with a number, such as two, of heat treatment units which are arranged horizontally and which load wafer boats containing wafers from below. A wafer delivery section is provided to correspond to each of the two heat treatment units, and wafers are conveyed by a wafer transfer mechanism between the wafer delivery section and the heat treatment units. Carrier accommodation racks are provided in upper and lower positions across the two heat treatment units in such a manner that they can be used in common for both wafer delivery sections. The carrier transfer mechanism is provided in front of the two carrier accommodation racks in order to convey carriers containing wafers between carrier stages at front portions of the wafer heat treatment apparatus, the two carrier accommodation racks, and the two wafer delivery sections. By making the carrier accommodation racks, the carrier transfer mechanism, and the carrier stages to be used in common for the two heat treatment units, the number of carrier accommodation positions can be increased for a single heat treatment unit, the carriers can be used more efficiently, and the area taken up by the apparatus can be reduced.