This invention aims to form uniformly and effectively an ultra thin chemical adsorbing film having an excellent water repellent property, oil repellent property, and contamination-proof property on the surface of a substrate by chemically adsorbing in a gas phase atmosphere. A chemically adsorbed film can be formed on any type of substrate and in a short time by chemically adsorbing a chlorosilane based surface-active agent on the surface of a substrate having active hydrogen groups. Further, a chemically adsorbed monomolecular film, or a polymer film, can be formed on any type of substrate and in a short time by forming a siloxane based monomolecular film or a polysiloxane adsorbed film in a gas atmosphere having a chlorosilane based surface-active agent having plurality of chlorosilyl groups.