A semiconductor device manufacturing method capable of making in-plane temperature distribution on a wafer uniform at heat treatment time. Before heat treatment is performed by irradiating the wafer with lamp light from the side of a device formed area where semiconductor devices are to be formed, an SiN film with certain thickness the reflection factor of which is equal to the average reflection factor of the device formed area is formed in an edge portion outside the device formed area. By doing so, reflection factors on the surface of the wafer irradiated with lamp light can be made uniform and uniform temperature distribution on the wafer can be obtained at heat treatment time. As a result, in-plane variations in the characteristics of semiconductor devices on the wafer can be made small and high-quality semiconductor devices can be manufactured.