PURPOSE: To enable the permeation of the impurities from a glass substrate to be satisfactorily avoided while an excellent quality polysilicon thin film to be formed.

CONSTITUTION: A silicon nitride thin film (lower side underneath layer) 2 is deposited on the surface while a silicon oxide thin film (upper side underneath layer) 3 is deposited on the layer 2 and then an amorphous silicon thin film 4 is deposited on the layer 3. Next, the whole surface is irradiated with excimer laser beams (a) to crystallize the amorphous silicon thin film 4 for the formation of a polysilicon thin film 5. In such a constitution, the lower side underneath layer is composed of the silicon nitride thin film 2 in the minute structure thereby enabling the permeation of the impurities from the glass substrate 1 to be satisfactorily avoided while the upper side underneath layer 3 is composed of the silicon oxide thin film 3 in relatively low thermal conductivity thereby enabling the excellent quality polysilicon thin film 5 in large particle diameter to be formed.