a heat - treating method and apparatus are disclosed which are employed for the manufacture of , for example , a semiconductor device . a substrate to be treated is set in a floating state with an air space defined relative to an underlying opposite surface . a ring - like temperature security member is located with a small gap left relative to an outer periphery of the substrate . the heating of the substrate is achieved in this state . infrared lamps are selected as a heating source and the surface underlying the substrate provides a reflection surface . the rear surface of the substrate is heated by reflection light travelling across the aforementioned air space . a heat dissipation from the marginal edge portion of the substrate is prevented by the temperature security member to obtain uniformity of heat over the whole area of the substrate . the presence of the air space ensures the ready loading and unloading of the substrate and hence the ready operability . it is also possible to prevent generation of dust resulting from a frictional contact between the substrate and the opposite surface .