A spark plug used for internal combustion engines, comprising: - a metal shell including a through hole that extends in an axial direction; and a metal convex portion that radially inwardly projects toward the through hole, wherein the metal convex portion is constituted by an inner circumferential face having a smallest inner diameter portion, a rearward face positioned on a rear end side of the inner circumferential face, and a forward face positioned on a front end side of the inner circumferential face; - an insulator including an axial bore that extends in the axial direction; on its outer circumferential face a first taper portion, a second taper portion and a base between the first and second taper portions (14, 36) and disposed on a front end side of the first taper portion, wherein the second insulator taper portion is positioned on a front end side of the base and has a contracted outer diameter toward a front end thereof; and an insulator front end portion that extends from the front end of the second taper portion toward the front end side and has an uniform outer diameter or an outer diameter smaller than that of the front end of the second insulator taper portion; wherein the base with which the insulator is provided faces the inner circumferential face of the metal convex portion in a proximity status in order to smoothly conduct heat from the insulator to the metal shell; wherein the first taper portion with which the insulator is provided is fixed by the rearward face of the metal convex portion of the metal shell; and wherein a very-end portion on the front end side of the inner circumferential face of the convex portion of the metal shell faces the base with which the insulator is provided; - a center electrode accommodated and held in the axial bore of the insulator; and - a ground electrode provided in a front end portion of the metal shell so that a front end portion of the ground electrode faces the front end face of the center electrode, and forming a spark discharge gap with a front end portion of the center electrode, characterized in that the spark plug satisfies the following representations to, G ≤ A − B / 2; A ≥ 7.3; and 2 ≤ XX ≤ 4, where "G" is the spark discharge gap measured in mm, where "A" is an inner diameter measured in mm of the through hole on a front end side with respect to the forward face of the convex portion of the metal shell, where "B" is an outer diameter measured in mm of a border between the second taper portion of the insulator and the insulator front end portion, and where "XX" is a length measured in mm in the axial direction from the very-end portion of the inner circumferential face of the convex portion of the metal shell to the border between the second taper portion of the insulator and the insulator front end portion. - a metal shell including a through hole that extends in an axial direction; and a metal convex portion that radially inwardly projects toward the through hole, wherein the metal convex portion is constituted by an inner circumferential face having a smallest inner diameter portion, a rearward face positioned on a rear end side of the inner circumferential face, and a forward face positioned on a front end side of the inner circumferential face; - an insulator including an axial bore that extends in the axial direction; on its outer circumferential face a first taper portion, a second taper portion and a base between the first and second taper portions (14, 36) and disposed on a front end side of the first taper portion, wherein the second insulator taper portion is positioned on a front end side of the base and has a contracted outer diameter toward a front end thereof; and an insulator front end portion that extends from the front end of the second taper portion toward the front end side and has an uniform outer diameter or an outer diameter smaller than that of the front end of the second insulator taper portion; wherein the base with which the insulator is provided faces the inner circumferential face of the metal convex portion in a proximity status in order to smoothly conduct heat from the insulator to the metal shell; wherein the first taper portion with which the insulator is provided is fixed by the rearward face of the metal convex portion of the metal shell; and wherein a very-end portion on the front end side of the inner circumferential face of the convex portion of the metal shell faces the base with which the insulator is provided; - a center electrode accommodated and held in the axial bore of the insulator; and - a ground electrode provided in a front end portion of the metal shell so that a front end portion of the ground electrode faces the front end face of the center electrode, and forming a spark discharge gap with a front end portion of the center electrode, where "G" is the spark discharge gap measured in mm, where "A" is an inner diameter measured in mm of the through hole on a front end side with respect to the forward face of the convex portion of the metal shell, where "B" is an outer diameter measured in mm of a border between the second taper portion of the insulator and the insulator front end portion, and where "XX" is a length measured in mm in the axial direction from the very-end portion of the inner circumferential face of the convex portion of the metal shell to the border between the second taper portion of the insulator and the insulator front end portion.