To obtain a steel cord excellent in rubber infiltrativity despite having compact structure made up of strand composed of plural filaments, and excellent in resistance to corrosion and buckling fatigue, and to obtain a pneumatic tire using these cords.

This steel cord is so designed that each of strands 2 is composed of three filaments 1 (hereafter, referred to as Fy) arranged in nearly parallel with one another, the strands 2 include strand A containing at least one filament 3 subjected to a pitch P (5Dt≤P≤40Dt; Dt is filament diameter) and preforming or postforming at a level of R (0.3Dt≤R≤:3Dt) in the form of wave, helix or their combination; and at least one of the three strands 2 adjacent to one another in triangular configuration represents the strand A, or a strand B composed of two Fys 1 arranged in nearly parallel with each other; the respective diameters of the Fy 1 substituted for the strand 2, strand A and strand B: Dt, Dd and Dm satisfy the following relationships: Dd≤Dm≤2Dd, and Dt≤Dm≤(1+2/31/2)Dt. The other objective pneumatic tire is obtained by using these steel chords in the belt of the tire.