Disclosed herein is a rubber-reinforcing steel wire which comprises: a steel wire; and a coating formed on the surface of said wire and containing a polymer of a triazine-thiol derivative represented by the following general formula: ##STR1## where R is --OR', --SR', --NHR', or --N(R').sub.2 ; R' is H, alkyl group, alkenyl group, phenyl group, phenylalkyl group, alkylphenyl group, or a cycloalkyl group, and M is H, Na, Li, K, 1/2Mg, 1/2Ba, 1/2Ca, primary, secondary or tertiary aliphatic amine, quaternary ammonium salt, or phosphonium salt.