A semiconductor wafer, on the surface of which a silicon dioxide base material and an amorphous silicon thin film are formed in this order, is carried into a chamber. An insulated gate bipolar transistor (IGBT) is connected with a power supply circuit to a flash lamp, and the IGBT makes an energization period to the flash lamp to be 0.01 millisecond or more and 1 millisecond or less, consequently making a flash light irradiation time to be 0.01 millisecond or more and 1 millisecond or less. Since a flash heat treatment is performed with a remarkably short flash light irradiation time, the excessive heating of the thin film of amorphous silicon is suppressed and harmful influence such as the exfoliation of the film is prevented.