Flash lamps connected to short-pulse circuits and flash lamps connected to long-pulse circuits are alternately arranged in a line. The duration of light emission from the flash lamps connected to the long-pulse circuits is longer than the duration of light emission from the flash lamps connected to the short-pulse circuits. A superimposing of a flash of light with a high peak intensity from the flash lamps that emit light for a short time and a flash of light with a gentle peak from the flash lamps that emit light for a long time can increase the temperature of even a deep portion of a substrate to an activation temperature or more without heating a shallow portion near the substrate surface more than necessary. This achieves the activation of deep junctions without causing substrate warpage or cracking.