To provide a steel wire for reinforcing a rubber product capable of making secular adhesiveness with the lapse of time satisfactory while satisfactorily maintaining the initial adhesiveness in vulcanized adhesion.

In a steel wire 10 for reinforcing a rubber product, a Cu plating layer and a Zn plating layer are successively formed on a surface of a steel wire 11, and thereafter, diffusion heat treatment is performed at 200 to 500°C to alloy Cu and Zn, thus a brass plating layer 14 is formed. A Cu composition in the brass plating layer 14 is restricted to 35 to 55 wt.%, and the crystal structure is made into the one of a γ phase simple substance, thus the progression of cross-linking reaction can be suppressed. The time-dependent adhesion with the lapse of time is made satisfactory while satisfactorily maintaining the initial adhesiveness with a rubber product as in the past without damaging the wire drawability.