A spark plug comprising: a rod-shaped center electrode extending in an axial direction; an insulator provided at an outer periphery of the center electrode; and a metal shell provided at an outer periphery of the insulator, the metal shell including: a tool engaging section extending in an outer peripheral direction, wherein a cross-section of the tool engaging section crossing at right angles to the axial direction has a polygonal-shape; a body section extending in the outer peripheral direction; and a groove section formed between the tool engaging section and the body section, and having a bulge which bulges in the outer peripheral direction and in an inner peripheral direction, characterized in that: wherein, when a portion of the groove section having a largest outer diameter is a first section, a thinnest portion of the groove section in the radial direction from the first section to the body section is a second section, and a portion of the groove section having a thickness the same as that of the first section in the radial direction at the body section is a third section, a relation between a thickness A of the second section in the radial direction at the cross-section including the axial direction and a radius of curvature R of an outer surface of the metal shell that continues from the second section to the third section satisfies R × A ≥ 0.21 mm 2. a rod-shaped center electrode extending in an axial direction; an insulator provided at an outer periphery of the center electrode; and a metal shell provided at an outer periphery of the insulator, the metal shell including: a tool engaging section extending in an outer peripheral direction, wherein a cross-section of the tool engaging section crossing at right angles to the axial direction has a polygonal-shape; a body section extending in the outer peripheral direction; and a groove section formed between the tool engaging section and the body section, and having a bulge which bulges in the outer peripheral direction and in an inner peripheral direction, characterized in that: wherein, when a portion of the groove section having a largest outer diameter is a first section, a thinnest portion of the groove section in the radial direction from the first section to the body section is a second section, and a portion of the groove section having a thickness the same as that of the first section in the radial direction at the body section is a third section, a relation between a thickness A of the second section in the radial direction at the cross-section including the axial direction and a radius of curvature R of an outer surface of the metal shell that continues from the second section to the third section satisfies R × A ≥ 0.21 mm 2. a tool engaging section extending in an outer peripheral direction, wherein a cross-section of the tool engaging section crossing at right angles to the axial direction has a polygonal-shape; a body section extending in the outer peripheral direction; and a groove section formed between the tool engaging section and the body section, and having a bulge which bulges in the outer peripheral direction and in an inner peripheral direction, characterized in that: wherein, when a portion of the groove section having a largest outer diameter is a first section, a thinnest portion of the groove section in the radial direction from the first section to the body section is a second section, and a portion of the groove section having a thickness the same as that of the first section in the radial direction at the body section is a third section, a relation between a thickness A of the second section in the radial direction at the cross-section including the axial direction and a radius of curvature R of an outer surface of the metal shell that continues from the second section to the third section satisfies R × A ≥ 0.21 mm 2. wherein, when a portion of the groove section having a largest outer diameter is a first section, a thinnest portion of the groove section in the radial direction from the first section to the body section is a second section, and a portion of the groove section having a thickness the same as that of the first section in the radial direction at the body section is a third section, a relation between a thickness A of the second section in the radial direction at the cross-section including the axial direction and a radius of curvature R of an outer surface of the metal shell that continues from the second section to the third section satisfies R × A ≥ 0.21 mm 2.