A spark plug comprising: an insulator having an axial hole extending along an axial line; a metal terminal member having an insert portion accommodated in the axial hole and a depression formation zone existing on an outer circumferential surface of the insert portion and having a plurality of depressions; and a metallic shell accommodating a forward portion of the insulator therein to thereby hold the insulator, the spark plug satisfying the following conditions: the insert portion has a length H of 35 mm or more along the axial line; the depression formation zone has a length F of 13 mm or more along the axial line; the insert portion has a smooth surface zone on its outer circumferential surface; and a ratio (A/B) between diameter A of a forward end of the insert portion and inside diameter B of the insulator measured at the forward end satisfies a relational expression 0.9 ≤ A/B ≤ 0.98; the spark plug being characterized in that the insert portion has a Vickers hardness of 150 Hv or more to 350 Hv or less as measured at the center of a cross section of the insert portion cut along a direction orthogonal to the axial line; the insulator has a first inner circumferential surface having the inside diameter B, a second inner circumferential surface having a diameter greater than the inside diameter B and disposed rearward of the first inner circumferential surface and the metallic shell, and a stepped portion connecting the first inner circumferential surface and the second inner circumferential surface, and at least a portion of the depression formation zone is disposed in a space surrounded by the stepped portion. an insulator having an axial hole extending along an axial line; a metal terminal member having an insert portion accommodated in the axial hole and a depression formation zone existing on an outer circumferential surface of the insert portion and having a plurality of depressions; and a metallic shell accommodating a forward portion of the insulator therein to thereby hold the insulator, the spark plug satisfying the following conditions: the insert portion has a length H of 35 mm or more along the axial line; the depression formation zone has a length F of 13 mm or more along the axial line; the insert portion has a smooth surface zone on its outer circumferential surface; and a ratio (A/B) between diameter A of a forward end of the insert portion and inside diameter B of the insulator measured at the forward end satisfies a relational expression 0.9 ≤ A/B ≤ 0.98; the insert portion has a length H of 35 mm or more along the axial line; the depression formation zone has a length F of 13 mm or more along the axial line; the insert portion has a smooth surface zone on its outer circumferential surface; and a ratio (A/B) between diameter A of a forward end of the insert portion and inside diameter B of the insulator measured at the forward end satisfies a relational expression 0.9 ≤ A/B ≤ 0.98; the spark plug being characterized in that the insert portion has a Vickers hardness of 150 Hv or more to 350 Hv or less as measured at the center of a cross section of the insert portion cut along a direction orthogonal to the axial line; the insulator has a first inner circumferential surface having the inside diameter B, a second inner circumferential surface having a diameter greater than the inside diameter B and disposed rearward of the first inner circumferential surface and the metallic shell, and a stepped portion connecting the first inner circumferential surface and the second inner circumferential surface, and at least a portion of the depression formation zone is disposed in a space surrounded by the stepped portion.