When a semiconductor film is irradiated with laser light, the semiconductor film is instantaneously melted and expand locally. In order to reduce internal stress generated by this expansion, strain is locally generated in the semiconductor film. Accordingly, a variation is caused among portions with strain and portions without strain, and a variation is caused also by a difference in extent of strain.According to the present invention, after laser light irradiation, an oxide film (referred to as a chemical oxide) is formed by using a solution containing ozone (typically, ozone water) to form an oxide film of 1 to 10 nm in total, and further, a heat treatment for reducing strain of a semiconductor film (a heat treatment of heating the semiconductor film instantaneously to approximately 400 to 1000.degree. C.) is performed.