A spark plug comprising: a cylindrical metal shell extending in an axial direction; a cylindrical insulator that is held in the metal shell and includes a leading end exposed from a leading end of the metal shell; a center electrode that is arranged in the insulator so that a leading end of the center electrode is exposed from the leading end of the insulator; and a ground electrode that includes a base end welded to a leading end surface of the metal shell so as to extend from the leading end of the metal shell (aa), a spark discharge gap (g) being formed between a leading end of the ground electrode and the leading end of the center electrode, wherein the base end of the ground electrode is welded to the leading end surface of the metal shell, the base end being protruded radially outward beyond an outer circumferential surface of the leading end of the metal shell and/or radially inward beyond an inner circumferential surface of the leading end of the metal shell, and wherein where S2 represents a sectional area of the welded part between the metal shell and the ground electrode, the sectional area S2 being cut off by a plane including the leading end surface of the metal shell, and S represents a sectional area of the ground electrode, the sectional area S being cut off by a plane which passes to a most leading end of a boundary in the axial direction between the ground electrode and the welded part and is perpendicular to the axial direction, a relation of S2≧1.1S is satisfied, wherein a relation between a radial thickness t of the leading end of the metal shell and a thickness T of the base end of the ground electrode is t<T, and wherein the ground electrode is weld to the metal shell so that an amount of outward protrusion that the base end of the ground electrode is protruded radially outward beyond the outer circumferential surface of the leading end of the metal shell is greater than an amount of inner protrusion that the base end of the ground electrode is protruded radially inward beyond the inner circumferential surface of the leading end of the metal shell. a cylindrical metal shell extending in an axial direction; a cylindrical insulator that is held in the metal shell and includes a leading end exposed from a leading end of the metal shell; a center electrode that is arranged in the insulator so that a leading end of the center electrode is exposed from the leading end of the insulator; and a ground electrode that includes a base end welded to a leading end surface of the metal shell so as to extend from the leading end of the metal shell (aa), a spark discharge gap (g) being formed between a leading end of the ground electrode and the leading end of the center electrode, wherein the base end of the ground electrode is welded to the leading end surface of the metal shell, the base end being protruded radially outward beyond an outer circumferential surface of the leading end of the metal shell and/or radially inward beyond an inner circumferential surface of the leading end of the metal shell, and wherein where S2 represents a sectional area of the welded part between the metal shell and the ground electrode, the sectional area S2 being cut off by a plane including the leading end surface of the metal shell, and S represents a sectional area of the ground electrode, the sectional area S being cut off by a plane which passes to a most leading end of a boundary in the axial direction between the ground electrode and the welded part and is perpendicular to the axial direction, a relation of S2≧1.1S is satisfied, wherein a relation between a radial thickness t of the leading end of the metal shell and a thickness T of the base end of the ground electrode is t<T, and wherein the ground electrode is weld to the metal shell so that an amount of outward protrusion that the base end of the ground electrode is protruded radially outward beyond the outer circumferential surface of the leading end of the metal shell is greater than an amount of inner protrusion that the base end of the ground electrode is protruded radially inward beyond the inner circumferential surface of the leading end of the metal shell.