A spark plug for an internal combustion engine, comprising: a rod-shaped center electrode that extends in an axial direction of the spark plug; a tubular insulator having an axial hole that extends in said axial direction of the spark plug in which said center electrode is received; a tubular metal shell disposed along an outer circumference of said insulator; and a ground electrode which extends from a tip end portion of said metal shell and is disposed in such a bent state that a tip end portion thereof is opposed to said center electrode; wherein a tip end portion of said center electrode and the tip end portion of said ground electrode cooperate to define a gap therebetween in which spark discharge is provided along said axial direction of the spark plug, wherein said ground electrode is formed with a narrowed width portion at the tip end portion thereof which has a substantially uniform width smaller than an outer diameter of a tip end surface of said center electrode, and a widened width portion which extends between a base end portion of said ground electrode and said narrowed width portion and has a width larger than the width of said narrowed width portion, wherein said narrowed width portion has a base end that is located offset from the tip end surface of said center electrode toward a root side of said ground electrode when viewed from a tip end side of the spark plug in said axial direction of the spark plug, wherein said base end of the narrowed width portion is located offset from a center of said gap toward the tip end side of the spark plug in said axial direction of the spark plug, wherein said narrowed width portion comprises side edges formed between a surface opposed to said center electrode and side surfaces disposed adjacent to the opposed surface, and said center electrode comprises inner edges (51 E) each being a part of a tip end edge formed between the tip end surface of said center electrode and an outer circumferential surface of said center electrode, said inner edges (51 E) each being disposed between said side edges of said narrowed width portion when viewed from the tip end side of the spark plug in said axial direction of the spark plug, wherein a maximum distance F extending between said side edges of said narrowed width portion and said inner edges (51 E) of said center electrode along a direction perpendicular to a center axis of said narrowed width portion and a distance G of said gap extending along said axial direction of the spark plug satisfy a relation expressed by the formula: F ≤ 1.25 G. a rod-shaped center electrode that extends in an axial direction of the spark plug; a tubular insulator having an axial hole that extends in said axial direction of the spark plug in which said center electrode is received; a tubular metal shell disposed along an outer circumference of said insulator; and a ground electrode which extends from a tip end portion of said metal shell and is disposed in such a bent state that a tip end portion thereof is opposed to said center electrode; wherein a tip end portion of said center electrode and the tip end portion of said ground electrode cooperate to define a gap therebetween in which spark discharge is provided along said axial direction of the spark plug, wherein said ground electrode is formed with a narrowed width portion at the tip end portion thereof which has a substantially uniform width smaller than an outer diameter of a tip end surface of said center electrode, and a widened width portion which extends between a base end portion of said ground electrode and said narrowed width portion and has a width larger than the width of said narrowed width portion, wherein said narrowed width portion has a base end that is located offset from the tip end surface of said center electrode toward a root side of said ground electrode when viewed from a tip end side of the spark plug in said axial direction of the spark plug, wherein said base end of the narrowed width portion is located offset from a center of said gap toward the tip end side of the spark plug in said axial direction of the spark plug, wherein said narrowed width portion comprises side edges formed between a surface opposed to said center electrode and side surfaces disposed adjacent to the opposed surface, and said center electrode comprises inner edges (51 E) each being a part of a tip end edge formed between the tip end surface of said center electrode and an outer circumferential surface of said center electrode, said inner edges (51 E) each being disposed between said side edges of said narrowed width portion when viewed from the tip end side of the spark plug in said axial direction of the spark plug, wherein a maximum distance F extending between said side edges of said narrowed width portion and said inner edges (51 E) of said center electrode along a direction perpendicular to a center axis of said narrowed width portion and a distance G of said gap extending along said axial direction of the spark plug satisfy a relation expressed by the formula: F ≤ 1.25 G.