A spark plug comprising: a tubular insulator extending along and centered on an axis; a tubular metal shell secured to an outer peripheral surface of the insulator by crimping, the tubular metal shell including an inner peripheral surface and being filled up with powder for sealing between the outer peripheral surface of the insulator and the inner peripheral surface, the metal shell including a tool engagement portion and a crimped lid, the tool engagement portion overhanging in a polygonal shape in an outer circumferential direction, the crimped lid being disposed at an end portion of the metal shell coupled to the tool engagement portion, the end portion being bent toward the outer peripheral surface of the insulator by crimping, the powder being filled between the crimped lid and the insulator; and an annular ring member that contacts the inner peripheral surface of the crimped lid of the metal shell and the outer peripheral surface of the insulator, characterized in that a relationship between a length L and a thickness t satisfies 2.50 ≤ L/t ≤ 3.10, the length L being along a shape of the crimped lid from an end face of the tool engagement portion to the insulator in a planar surface YZ that passes through the axis, wherein the length L is along a curved line (Ps-Pe) in the planar surface YZ, the curved line (Ps-Pe) starting at a point Ps and reaching a point Pe, and passing through the centers of a plurality of virtual circles (C1, Cn), wherein each circle (Ck) of the plurality of virtual circles (C1, Cn) being with a minimum diameter such that, in the planar surface YZ, each circle (Ck) passes through a respective point (Pdk) on the outline outside of the crimped lid and contacts the outline inside of the crimped lid, wherein the point Ps is a contact point of a 0 th virtual circle and a planar surface PLb, the 0 th virtual circle contacting the outline outside of the crimped lid and the outline inside of the crimped lid, and wherein the planar surface PLb passes through the end face, and wherein the point Pe is a contact point of an e th virtual circle (Ce) of the plurality of virtual circles (C1, Cn) and the end portion; and wherein the thickness t is a thickness at an intermediate portion of the crimped lid, wherein the thickness t is the diameter of a virtual circle Cm, centered on a point Pm, of minimum diameter that contacts the outline outside of the crimped lid and the outline inside of the crimped lid, wherein the point Pm is a length L/2 from the point Ps on the curved line (Ps-Pe); wherein a relationship between a minimum outer diameter d of the insulator and a maximum inner diameter D of the metal shell in a filling-up area satisfies 1.12 ≤ D/d ≤ 1.16, the minimum outer diameter d being in the filling-up area where the powder is filled between the metal shell and the insulator. a tubular insulator extending along and centered on an axis; a tubular metal shell secured to an outer peripheral surface of the insulator by crimping, the tubular metal shell including an inner peripheral surface and being filled up with powder for sealing between the outer peripheral surface of the insulator and the inner peripheral surface, the metal shell including a tool engagement portion and a crimped lid, the tool engagement portion overhanging in a polygonal shape in an outer circumferential direction, the crimped lid being disposed at an end portion of the metal shell coupled to the tool engagement portion, the end portion being bent toward the outer peripheral surface of the insulator by crimping, the powder being filled between the crimped lid and the insulator; and an annular ring member that contacts the inner peripheral surface of the crimped lid of the metal shell and the outer peripheral surface of the insulator, characterized in that a relationship between a length L and a thickness t satisfies 2.50 ≤ L/t ≤ 3.10, the length L being along a shape of the crimped lid from an end face of the tool engagement portion to the insulator in a planar surface YZ that passes through the axis, wherein the length L is along a curved line (Ps-Pe) in the planar surface YZ, the curved line (Ps-Pe) starting at a point Ps and reaching a point Pe, and passing through the centers of a plurality of virtual circles (C1, Cn), wherein each circle (Ck) of the plurality of virtual circles (C1, Cn) being with a minimum diameter such that, in the planar surface YZ, each circle (Ck) passes through a respective point (Pdk) on the outline outside of the crimped lid and contacts the outline inside of the crimped lid, wherein the point Ps is a contact point of a 0 th virtual circle and a planar surface PLb, the 0 th virtual circle contacting the outline outside of the crimped lid and the outline inside of the crimped lid, and wherein the planar surface PLb passes through the end face, and wherein the point Pe is a contact point of an e th virtual circle (Ce) of the plurality of virtual circles (C1, Cn) and the end portion; and wherein the thickness t is a thickness at an intermediate portion of the crimped lid, wherein the thickness t is the diameter of a virtual circle Cm, centered on a point Pm, of minimum diameter that contacts the outline outside of the crimped lid and the outline inside of the crimped lid, wherein the point Pm is a length L/2 from the point Ps on the curved line (Ps-Pe); wherein a relationship between a minimum outer diameter d of the insulator and a maximum inner diameter D of the metal shell in a filling-up area satisfies 1.12 ≤ D/d ≤ 1.16, the minimum outer diameter d being in the filling-up area where the powder is filled between the metal shell and the insulator.