A spark plug comprising: a tubular insulator (3, 203, 303, 403) having an axial hole extending in the direction of an axial line, the insulator (3, 203, 303, 403) including a step portion which is disposed on an outer circumference of the insulator (3, 203, 303, 403) and whose outer diameter decreases toward a forward end of the insulator (3, 203, 303, 403); a center electrode (4, 204, 304, 404) disposed at a forward end of the axial hole; and a tubular metallic shell disposed around the insulator (3, 203, 303, 403), the step portion being engaged with an inner circumference of the metallic shell, wherein the insulator (3, 203, 303, 403) includes a leg portion located forward of and adjacent to the step portion, wherein the leg portion includes a first section (41, 51, 61, 71) which is located forward of and adjacent to the step portion and whose outer diameter decreases from a base end of the first section (41, 51, 61, 71) toward a forward end thereof, a second section (42, 52, 62, 72) which is located forward of and adjacent to the first section (41, 51, 61, 71) and whose outer diameter decreases from a base end of the second section (42, 52, 62, 72) toward a forward end thereof, wherein, in a cross section obtained by cutting the leg portion and including the axial line, a line (T 42, T 52, T 62, T 72) representing an outer circumferential surface of the second section (42, 52, 62, 72) is located inward of a first straight line (L 41, L 51, L 61, L 71) passing through base and forward ends of an outer circumferential surface of the first section (41, 51, 61, 71), and a third section (43, 53, 63, 73) which is located forward of and adjacent to the second section (42, 52, 62, 72) and whose outer diameter is equal to or less than a forward end outer diameter Ds of the second section (42, 52, 62, 72) over the entire third section (43, 53, 63, 73), wherein, in the cross section, a line (T 43, T 53, T 63, T 73) representing at least an outer circumferential surface of a rear end portion of the third section (43, 53, 63, 73), which rear end portion is connected to the forward end of the second section (42, 52, 62, 72), is located outward of a second straight line (L 42, L 52, L 62, L 72) passing through base and forward ends of an outer circumferential surface of the second section (42, 52, 62, 72), characterized in that an outer circumferential surface of the leg portion has an arithmetic mean roughness Ra of 0.5 μm or less, a forward end angle θ between the first straight line and the second straight line is θ ≥ 15°, and an expression (Dk - Ds)/2 which represents the relation between the forward end outer diameter Ds and a base end outer diameter Dk of the second section (42, 52, 62, 72) satisfies 5 μm ≤ (Dk - Ds)/2 ≤ 200 μm. a tubular insulator (3, 203, 303, 403) having an axial hole extending in the direction of an axial line, the insulator (3, 203, 303, 403) including a step portion which is disposed on an outer circumference of the insulator (3, 203, 303, 403) and whose outer diameter decreases toward a forward end of the insulator (3, 203, 303, 403); a center electrode (4, 204, 304, 404) disposed at a forward end of the axial hole; and a tubular metallic shell disposed around the insulator (3, 203, 303, 403), the step portion being engaged with an inner circumference of the metallic shell, wherein the insulator (3, 203, 303, 403) includes a leg portion located forward of and adjacent to the step portion, wherein the leg portion includes a first section (41, 51, 61, 71) which is located forward of and adjacent to the step portion and whose outer diameter decreases from a base end of the first section (41, 51, 61, 71) toward a forward end thereof, a second section (42, 52, 62, 72) which is located forward of and adjacent to the first section (41, 51, 61, 71) and whose outer diameter decreases from a base end of the second section (42, 52, 62, 72) toward a forward end thereof, wherein, in a cross section obtained by cutting the leg portion and including the axial line, a line (T 42, T 52, T 62, T 72) representing an outer circumferential surface of the second section (42, 52, 62, 72) is located inward of a first straight line (L 41, L 51, L 61, L 71) passing through base and forward ends of an outer circumferential surface of the first section (41, 51, 61, 71), and a third section (43, 53, 63, 73) which is located forward of and adjacent to the second section (42, 52, 62, 72) and whose outer diameter is equal to or less than a forward end outer diameter Ds of the second section (42, 52, 62, 72) over the entire third section (43, 53, 63, 73), wherein, in the cross section, a line (T 43, T 53, T 63, T 73) representing at least an outer circumferential surface of a rear end portion of the third section (43, 53, 63, 73), which rear end portion is connected to the forward end of the second section (42, 52, 62, 72), is located outward of a second straight line (L 42, L 52, L 62, L 72) passing through base and forward ends of an outer circumferential surface of the second section (42, 52, 62, 72), characterized in that an outer circumferential surface of the leg portion has an arithmetic mean roughness Ra of 0.5 μm or less, a forward end angle θ between the first straight line and the second straight line is θ ≥ 15°, and an expression (Dk - Ds)/2 which represents the relation between the forward end outer diameter Ds and a base end outer diameter Dk of the second section (42, 52, 62, 72) satisfies 5 μm ≤ (Dk - Ds)/2 ≤ 200 μm.