A spark plug comprising: an insulator having an axial hole formed therein in a direction of an axis of the spark plug; a center electrode disposed in the axial hole, with a front end portion of the center electrode protruding from a front end of the insulator; a resistor disposed in the axial hole at a position closer to a rear end of the spark plug than the center electrode; and a seal member disposed in the axial hole at a position between the resistor and the center electrode so as to connect the resistor and the center electrode to each other, wherein the insulator includes: an inner-diameter decreasing portion having an inner diameter decreasing toward a front end of the spark plug; and a small inner-diameter portion located closer to the front end of the spark plug than the inner-diameter decreasing portion; and the center electrode includes a head portion located at a position closer to the rear end of the spark plug than the small inner-diameter portion of the insulator and supported on the inner-diameter decreasing portion of the insulator; characterized in that the spark plug satisfies the following conditions: 1.8 mm ≤ L; and Cp ≤ 11 mm where, assuming a region of the insulator extending from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member in the direction of the axis as a specific region, L is a length of the specific region in the direction of the axis; 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 a value given by L/log(D2/D1). an insulator having an axial hole formed therein in a direction of an axis of the spark plug; a center electrode disposed in the axial hole, with a front end portion of the center electrode protruding from a front end of the insulator; a resistor disposed in the axial hole at a position closer to a rear end of the spark plug than the center electrode; and a seal member disposed in the axial hole at a position between the resistor and the center electrode so as to connect the resistor and the center electrode to each other, wherein the insulator includes: an inner-diameter decreasing portion having an inner diameter decreasing toward a front end of the spark plug; and a small inner-diameter portion located closer to the front end of the spark plug than the inner-diameter decreasing portion; and the center electrode includes a head portion located at a position closer to the rear end of the spark plug than the small inner-diameter portion of the insulator and supported on the inner-diameter decreasing portion of the insulator; characterized in that the spark plug satisfies the following conditions: 1.8 mm ≤ L; and Cp ≤ 11 mm where, assuming a region of the insulator extending from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member in the direction of the axis as a specific region, L is a length of the specific region in the direction of the axis; 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 a value given by L/log(D2/D1).