A method for producing a semiconductor element which can form a deep P-type impurity region by a diffusion of aluminum. A porous alumina layer is first formed on a semiconductor substrate. Then, a diffusion-protective layer formed of a material having a large oxygen-diffusion-inhibiting ability such as Al.sub.2 O.sub.3 is formed on the porous alumina layer. Subsequently, aluminum ions are implanted in the porous alumina layer through the diffusion-protective layer. Thereafter, a heat treatment is performed to diffuse the aluminum of the aluminum ion-implanted region in the semiconductor substrate, and a P-type impurity region is formed. Alternatively, a porous alumina layer is formed on the semiconductor substrate, and an aluminum layer is then formed thereon. The diffusion-protective layer is formed on the aluminum layer, and a heat treatment is then performed, thereby diffusing the aluminum forming the aluminum layer in the semiconductor substrate, and a P-type impurity region is thus formed.