PROBLEM TO BE SOLVED: To provide a low cost method for surface-treating a metal base material, capable of sufficiently forming an aluminium diffused layer on a metal base material to be treated even when subjected to calorizing treatment under atmospheric atmosphere.SOLUTION: A method for surface-treating a metal base material comprises: the preparation step of preparing a vessel having the metal base material buried into a treatment agent 5 including an Fe-Al alloyed powder and an ammonium chloride powder and charged into the inside of the vessel; and the calorizing treatment step of heating the container to form an aluminium diffused layer on the surface layer part of the metal base material. The calorizing treatment step is performed under atmospheric atmosphere, and a dense film 5a is formed on an interface I between the treatment agent 5 and the atmosphere 70 before an aluminium diffused layer is formed and includes alumina obtained by the reaction of aluminium included in the treatment agent 5 with oxyg
