PURPOSE: To eliminate contaminant etc., without washing, eliminate an oxide film without damaging a substrate, and form a thin film without exposing to the air by eliminating organic contaminant, etc., stuck to the surface of the semiconductor substrate and then forming a thin film on the cleaned surface.

CONSTITUTION: A semiconductor substrate 13 is exposed to an atmosphere including oxidizing gas and a light is applied to the substrate 13 while heated to oxidize the surface of the substrate 13 by photochemical reaction, and eliminate harmful organic contaminant etc., 20 which may be stuck to the surface of the substrate 13. At the same time, an extremely thin oxide film 21 to take a harmful metal pollution layer or surface damaging layer thereinto is grown on the surface of the substrate 13. The oxide film 21 is eliminated by applying a light in an atmosphere including chloric gas to expose the cleaned surface of the substrate, and a thin film 22 is formed continuously on the cleaned surface without exposing to the air.