A very thin coating layer having a thickness of the order of 1-100 angstroms of a hydroxymethyl substituted phenol is applied to the surface of a metal material. The hydroxylmethyl substituted phenol, such as saligenin, is applied in the gaseous phase to the surface of the metal material maintained at a high temperature.By forming this ultra-thin coating layer, the heat bondability of a thermoplastic resin layer to the metal material can be effectively improved. This technique is advantageously used in various fields, for example, for production of bonded cans. The metal material comprises a steel plate substrate and a chromium-containing layer on the surface of the substrate, and the very thin layer is applied on the chromium-containing layer.