PURPOSE: To obtain a steel cord used as a reinforcing material for elastomer products such as pneumatic tires, and to improve the fatigue resistance of tires reinforced with the cord.

CONSTITUTION: At least a part of strands is made up of core consisting of a single steel filament and sheath seven steel filaments. The core filament is a wire which is so designed that the locus of its section perpendicular to the longer direction of the cord stands ellipsoidally preformed or postformed, having the following parameters: the ratio of the major axis (a) to minor axis (b) for the ellipsoid is 1.1≤(a/b)≤2.3; preformed or postformed rate Rc expressed by the ratio of the preformed or postformed amplitude Lc to the wire diameter (dc):(Lc/dc) is 0.42≤Rc≤1.86; and the preformed or postformed pitch Pc satisfies the relationship: 3.0(dc)/0.34≤Pc≤8.0(dc)/0.34.