In a heat treatment apparatus, a reflector is provided to cover a plurality of flash lamps arranged in an array for emitting a flash of light, and a cooling box is provided over the reflector. The cooling box has a buffer space incorporated therein, and a plurality of jet openings in communication with the buffer space are formed through a bottom surface of the cooling box and the reflector. The plurality of jet openings are positioned just over gaps between the plurality of flash lamps in the lamp array. Nitrogen gas ejected from the plurality of jet openings passes through the gaps between adjacent ones of the flash lamps in the lamp array, and is then blown against a lamp light radiation window. The flash lamps are effectively cooled down by the direct cooling using the nitrogen gas and the decrease in temperature of the lamp light radiation window.