Before a start of a treatment of a semiconductor wafer to be treated first in a lot, a dummy wafer is transported into a chamber, and an atmosphere including a helium gas having high thermal conductivity is formed. When the dummy wafer is heated with light irradiation from halogen lamps, heat transfer from the dummy wafer the temperature of which has increased occurs at an upper chamber window and a lower chamber window, with the helium gas as a heating medium. At the time when the semiconductor wafer to be treated first is transported into the chamber, the upper chamber window and the lower chamber window are heated, which makes a temperature history of all the semiconductor wafers in the lot uniform. It is thus possible to omit dummy running.