There is provided a steel cord including a plurality of untwisted core filaments of steel aligned in parallel, and a layer of sheath filaments of steel twisted around the core filaments so as to be unevenly distributed around the core filaments, wherein interstices between the filaments are maintained during vulcanization thereby achieving improved rubber penetration (sufficiently adhering rubber to the core filaments). Since the cross sectional length of the steel cord 10 is greater than the minimum cross sectional length, interstices A are maintained between sheath filaments 14 under the tension and pressure p of the surrounding rubber 16 applied to the steel cord 10 during vulcanization. Rubber 16 penetrates into the steel cord 10 through the interstices A, and sufficiently adhere to core filaments 12 to achieve high rubber penetration.