A reduction of unevenness of temperature increase at the end of heating work can be achieved without specific equipment, and with securing shortening of working time, which is an advantage of a bulk heating. When a region to be heated of an article made of a thin sheet is heated inductively so that the whole region thereof reaches a temperature equal to or more than a target temperature using an inductive portion of a heating inductor to which a high-frequency current is applied by a power supply device, by providing a period to stop or reduce temporarily the power application of the high-frequency current to the heating inductor during the temperature increase, a temperature difference in the region to be heated is reduced so that the unevenness of the temperature increase at the end of the heating work is reduced by the step of reducing the temperature difference.