PURPOSE: To provide an element with an active layer having excellent crystallinity and to control leak current at a low level, by converting polycrystalline or amorphous silicon on an insulation film into single crystal silicon and converting the silicon in the region where an element isolating region is to be formed into porous silicon to selectively oxidize them.

CONSTITUTION: An SiO2 layer (insulation layer) 22 is formed on a P-type single crystal silicon substrate 21 having P type (100) plane orientation. An SiN layer 23 is formed thereon and patterned to provide a mask for etching the SiO2 layer 22. After the etching of the layer 22, polycrystalline silicon film 24 is deposited on the whole surface. A linear electron beam EB is applied to scan orthogonally to the edges of a recess provided in the film 24 so that the polycrystalline silicon layer 24 is converted into a single crystal silicon layer 24'. An SiN layer 25 is provided and patterned. Proton ions are implanted in the region where in element isolating region is to be formed for providing a porous silicon layer 26, and the layer 26 is converted into SiO2 26' by means of the oxidation process.