Abstract: To improve characteristics of a semiconductor device. An element isolation region is etched by using a photoresist film as a mask, and thereby a p-type well that is a layer under the element isolation region is exposed. Thereafter, deposit over a surface of the photoresist film is etched. Then, a source region is formed by implanting impurity ions into the exposed p-type well by using the photoresist film as a mask, and thereafter, the photoresist film is removed. Thereby, it is possible to prevent a hardened layer from being formed due to injection of impurity ions into the deposit over the surface of the photoresist film. As a result, it is possible to suppress a popping phenomenon when the photoresist film is removed, so that it is possible to prevent a pattern of a gate and the like from being broken.