PURPOSE: To improve adhesion at a brass-plated steel wire to a rubber.

CONSTITUTION: To the brass-plated 2 surface, P of 1.0mg/m2 to 3.0mg/m2 is made to adhere and the surface roughness of the peripheral surface of a wire 1 constituting a boundary between the brass-plated layer and the wire is controlled to ≤(0.019+0.159d) μm Ra in relation to the diameter (d) of the wire. Thereby, a suitable amount of P is uniformly and finely distributed on the whole peripheral surface of the brass-plated wire and adhesion to various rubbers can be improved regardless of the kinds of the rubbers.