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.