To provide a steel cord having excellent resistance to corrosion propagation with hardly generating any separation and suitable for a pneumatic radial tire by using steel filaments having a specific range of diameter ratio to construct two-ply twisted cord of sheath-core structure.

This rubber material-reinforcing steel cord in which a ratio (a/b) of a short diameter (a) in a cord cross section to a large diameter (b) in the cross section is 63-100% is obtained by constructing a core 2 by using two steel filament 1 respectively having 0.10-0.40 mm diameter (dc) add twisting them at a twisting pitch of ≥30 mm in average, and constructing a two-ply twisted cord by forming a sheath 4 comprising 9-pieces of steel filaments 3 in which respective ratio (ds/dc) of a diameter (ds) to the diameter (dc) of the steel filament 1 is 35-80% and the (ds) is 0.10-0.37 mm around the core 2. The steel cord is used as a radial carcass and/or a belt part to form the objective pneumatic radial tire.