To provide an apparatus which can equalize the powers to be supplied to a plurality of lamps which constitute a light source for heating, and which can drive all the lamps near the rated output.

Between the lamps 28 and a light-incident window 14 of a heat treatment furnace 10, a group of lenses 44 which consists of a plurality of lenses 46 arranged two-dimensionally in parallel is interposed. Among the group of lenses, those disposed near the center are concave lenses, and a condensing degree of a plurality of lenses disposed around those near the center is made higher as gradually approaching toward the periphery.