PURPOSE: To effectively obtain a dense thin film (of low resistance for a conducting thin film and of high resistance for a insulating thin film) in forming a thin film on the surface of a substrate.

CONSTITUTION: When a thin film is formed on the surface of a substrate, the stage where insular substances are grown and united to form a continuous film is detected and based on the results, the optimum film forming condition in the initial process of film forming (the optimum energy of ions or neutral particles irradiated on a thin film) is found to obtain a dense thin film by using the condition early in the film forming.