Disclosed herein is a process for forming a single crystal silicon layer by heating a wafer, which is made of a single crystal silicon substrate and a starting silicon layer made of amorphous or polycrystalline silicon and provided on the silicon substrate, in accordance with the epitaxial growth technique. The process comprises providing a heat source which is formed of a plurality of tubular lamps provided side by side with their longitudinal axes extending substantially in parallel with one another in a second plane lying above and substantially in parallel with a first plane--in which the wafer is placed--and a tubular melting-lamp provided at a position between the first and second planes and with its longitudinal axis substantially in parallel with the longitudinal axes of the tubular lamps in the second plane; and moving the wafer in the first plane and in a direction perpendicular to the longitudinal axes of the plurality of tubular lamps and that of the tubular melting-lamp in a state that all the tubular lamps of the heat source, including the tubular melting-lamp, are lit on. The above process can convert the starting silicon layer in its entirety into a single crystal silicon layer in a relatively short period of time and without danger of damaging the wafer. The above process facilitates formation of single crystal silicon layers which make up SOI structures.