A heat treatment apparatus for oxidation having: an oven for oxidation having a heat treatment chamber having a plurality of slits through which fiber strands running horizontally leave or returned strands enter and capable of sending hot air vertically from above the fiber strands to allow the fiber strands to have oxidation, and a means for feeding hot air into the heat treatment chamber, and a plurality of returning rollers which are provided at the two outsides of the oven for oxidation and which return the fiber strands entering and leaving through said slits, into the oven for oxidation, wherein each gap formed between fiber strands and each side wall of heat treatment chamber parallel to the running direction of fiber strands running in the heat treatment chamber, or each gap formed between fiber strands and each channeling-preventing plate interposed between the side wall and the fiber strands in parallel to the running direction of fiber strands is set at 150 mm or less. The slits may be provided with a means for injecting hot air into the heat treatment chamber.