A silicon nitride film including stoichiometrically excessive silicon with respect to nitrogen is formed. The silicon nitride film may be formed by supplying dichlorosilane to a substrate under a condition where CVD (chemical vapor deposition) reaction is caused to form a silicon film including several or less atomic layers on the substrate, supplying ammonia to the substrate in a non-plasma atmosphere to thermally nitride the silicon film under a condition where the nitriding reaction of the silicon film by the ammonia is not saturated, and alternately repeating the supplying of dichlorosilane and the supplying of ammonia.