A spark plug (10, 90, 130) comprising: a center electrode including a nose portion that extends from a front side to a rear side along an axial line, and a flange portion that projects to a radially outer side from a rear end of the nose portion; a cylindrical insulator (40, 100, 140) in which a receiving portion supporting the flange portion is formed on an axial hole (41, 151, 161) formed along the axial line, and a step portion having a diameter that increases from the front side toward the rear side is formed on an outer circumferential surface; and a cylindrical metal shell in which a shelf portion supporting the step portion via a packing is formed on an inner circumferential surface and which is disposed at the radially outer side of the insulator (40, 100, 140), wherein a front end portion (45, 101, 141) of the insulator (40, 100, 140) disposed on the front side has a length L of less than or equal to 9 mm in an axial direction from a front end edge to a front end surface of the insulator (40, 100, 140), wherein the front end edge is an edge present on the frontmost side of a first contact surface of the step portion that the packing contacts with the first contact surface; the front end portion (45, 101, 141) includes an annular groove opened to the front side and formed around the axial line, the groove has a width of greater than or equal to 0.2 mm in a radial direction, and in a cross section including the axial line, a value D/L obtained by dividing, by the length L, a creepage distance D, from a position P on a frontmost side of a region in which a clearance distance between an outer surface of the front end portion (45, 101, 141) and the inner circumferential surface of the metal shell is less than or equal to 0.1 mm to a connection position between the outer surface of the front end portion (45, 101, 141) and the axial hole (41, 151, 161), is greater than or equal to 1.1. a center electrode including a nose portion that extends from a front side to a rear side along an axial line, and a flange portion that projects to a radially outer side from a rear end of the nose portion; a cylindrical insulator (40, 100, 140) in which a receiving portion supporting the flange portion is formed on an axial hole (41, 151, 161) formed along the axial line, and a step portion having a diameter that increases from the front side toward the rear side is formed on an outer circumferential surface; and a cylindrical metal shell in which a shelf portion supporting the step portion via a packing is formed on an inner circumferential surface and which is disposed at the radially outer side of the insulator (40, 100, 140), wherein a front end portion (45, 101, 141) of the insulator (40, 100, 140) disposed on the front side has a length L of less than or equal to 9 mm in an axial direction from a front end edge to a front end surface of the insulator (40, 100, 140), wherein the front end edge is an edge present on the frontmost side of a first contact surface of the step portion that the packing contacts with the first contact surface; the front end portion (45, 101, 141) includes an annular groove opened to the front side and formed around the axial line, the groove has a width of greater than or equal to 0.2 mm in a radial direction, and in a cross section including the axial line, a value D/L obtained by dividing, by the length L, a creepage distance D, from a position P on a frontmost side of a region in which a clearance distance between an outer surface of the front end portion (45, 101, 141) and the inner circumferential surface of the metal shell is less than or equal to 0.1 mm to a connection position between the outer surface of the front end portion (45, 101, 141) and the axial hole (41, 151, 161), is greater than or equal to 1.1.