A spark plug has an insulator, a center electrode disposed in an axial hole, a resistor disposed in the axial hole and a seal member disposed between the resistor and the center electrode in the axial hole. The insulator includes an inner-diameter decreasing portion and a small inner-diameter portion. The center electrode includes a head portion supported on the inner-diameter decreasing portion of the insulator. The spark plug satisfies the following conditions: 1.8 mm≤L; and Cp≤11 mm where, assuming a region of the insulator from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member as a specific region, L is a length of the specific region; D1 is an average inner diameter of the axial hole within the specific region; D2 is an average outer diameter of the specific region; and Cp is L/log(D2/D1).