A spark plug includes an insulator having an axial hole, a metal terminal member having an insert portion and a depression formation zone existing on the insert portion and having depressions, and a metallic shell. The spark plug satisfies the following conditions: (1) the insert portion has a length H of 35 mm or more; (2) the depression formation zone has a length F of 13 mm or more; (3) the insert portion has a smooth surface zone on its outer circumferential surface; (4) the 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 the relational expression 0.9≦A/B≦0.98; and (5) Vickers hardness of the insert portion measured at the center of a cross section of the insert portion is 150 Hv or more to 350 Hv or less.