PURPOSE: To obtain a thin film of large crystal grain diameter by forming a seed crystal on a substrate and an amorphous material layer on the seed crystal and by crystallizing the amorphous material layer by heat treatment with the seed crystal as the crystal nucleus to form a thin film.

CONSTITUTION: A heat treatment is made in an inactive gas such as nitrogen to crystallize an amorphous silicon layer 3 with an island silicon single crystal 2 as the seed crystal. Thus, a polycrystalline silicon thin film 4 is formed over a substrate 1. This crystallization is as follows: crystal growth proceeds centering the seed crystal because of presence of the seed crystal serving as the crystal nucleus. Therefore, the smaller is number of first silicon single crystals 2, the smaller is the whole number of crystals: that is, the grain diameter of every crystal is larger. It follows that a polycrystalline silicon thin film 4 of 2-3μm large grain diameter is formed. That is, polycrystalline silicon of large grain diameter and high conductivity can be obtained by a simple device, a simple handling, and a continuous process.