Heat treatment method by light irradiation

A semiconductor wafer to be treated is placed on a susceptor made of quartz installed in a chamber, and is heated by light irradiation from halogen lamps. Before the first semiconductor wafer of a production lot is transported into the chamber, a preheating substrate is placed on the susceptor. Then, the preheating substrate is heated by light irradiation from the halogen lamps to preheat the susceptor. The susceptor is heated to a preheating temperature higher than a stable temperature when the semiconductor wafers of the production lot are continuously treated. This enables a structure in the chamber, other than the susceptor, to be preheated to a temperature during steady treatment of the semiconductor wafer in a short time, so that it is possible to eliminate dummy running for heating the structure in the chamber by applying heating treatment to a plurality of dummy wafers.