NOVELTY - The method involves performing i-th flash irradiation on a substrate i.e. disk-shaped semiconductor wafer (W), to heat a front surface of the substrate by discharging electrical charges stored in a capacitor through a flash lamp (FL), where i is a positive integer. The i+1 th flash irradiation is performed on the substrate by discharging electrical charges remaining in the capacitor through the flash lamp to reheat the front surface of the substrate before the temperature of the front surface of the substrate is equal to the temperature of a back surface of the substrate.    USE - Method for heating a substrate i.e. disk-shaped semiconductor wafer, of a flat panel display in an LCD apparatus, and a solar cell. Can also be used for heating a glass substrate, joining of metal and silicon, and for crystallization of polysilicon.    ADVANTAGE - The method enables reducing extraction of more energy remaining in the capacitor by consuming in prior iteration to improve the efficiency of heating of the front surface of the substrate resulting from the iteration of the flash irradiation. The method enables performing the i+1 th flash irradiation after the temperatures of the front and back surfaces of the semiconductor wafer is equal to the temperature of the back surface of the substrate, so that the temperature of the front surface at the time of the i+1 flash irradiation is decreased to a considerably low degree due to heat conduction from the front surface to the back surface so as to enable a relatively low degree of attained temperature of the front surface of the semiconductor wafer to be inevitably caused by the i+1 flash irradiation, thus increasing the attained temperature of the front surface of the semiconductor wafer to reduce the energy consumed during the i+1 flash irradiation even when an irradiation time period for the i+1 flash irradiation is made shorter than that for the i-th flash irradiation, and hence improving efficiency of heating of the front surface of the semiconductor wafer.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a heat treatment apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a longitudinal sectional view of a heat treatment apparatus.    Flash lamp (FL)    Disk-shaped semiconductor wafer (W)    Heat treatment apparatus (1)    Shutter plate (21)    Sliding drive mechanism (22)