Providing improved wear resistance to a metal material by hardening a surface layer and a surface layer hardening method. A base material is nitrided so that a metal material (40) has a surface layer hardened. The surface layer of the base material is formed with no nitrogen compound layer (40C), and the base material includes a region from a surface thereof to a depth of 78 μm, the region having a Vickers hardness higher than the base material by not less than 5%.