In forming a thin film of silver mirror on a surface of an object to be coated, a silver mirror reaction-treating solution including three solutions: an ammoniacal silver salt aqueous solution (I), an aqueous solution of caustic soda (IIa) and an aqueous solution of a carbohydrate-based reducing agent (IIb) such as glucose (fructose) are used. Immediately after the caustic soda aqueous solution (IIa) and the reducing agent aqueous solution (IIb) are mixed, the resulting mixed liquid and the ammoniacal silver salt aqueous solution (I) are independently and simultaneously sprayed onto the object. Alternatively, the mixed liquid (II) and the ammoniacal silver salt aqueous solution (I) are mixed together immediately upstream of a spraying nozzle, and the mixed liquid (III) is sprayed onto the object. Thereby, a thin film of silver mirror is formed in a thickness of around 0.01 to 0.03 mu m, for example, through depositing silver by a silver mirror reaction. <IMAGE>