A spark plug comprising: an insulator having an axial bore extending in a direction of an axis, a first inner circumferential surface extending at a forward side of the axial bore, a second inner circumferential surface extending at a rear side of the axial bore and having a diameter greater than that of the first inner circumferential surface, and a ledge connecting the first inner circumferential surface and the second inner circumferential surface; a center electrode having a head supported by the ledge and extending in a space surrounded by the second inner circumferential surface, and a circular columnar leg extending continuously from a forward end of the head in a space surrounded by the first inner circumferential surface; and a seal material charged into a space surrounded by the ledge, the second inner circumferential surface, and the head to thereby hold the center electrode in the axial bore; the spark plug being characterized in that: a position P1 on the center electrode is where an outside diameter of the center electrode begins to increase, beyond an average outside diameter H of the leg, from the leg toward a rear end of the center electrode, and when an axial distance E along the axis from the position P1 to a forward end of the center electrode is 15 mm or more, an axial distance A along the axis from the position P1 to the rear end of the center electrode is from 3.8 mm to 5.0 mm. an insulator having an axial bore extending in a direction of an axis, a first inner circumferential surface extending at a forward side of the axial bore, a second inner circumferential surface extending at a rear side of the axial bore and having a diameter greater than that of the first inner circumferential surface, and a ledge connecting the first inner circumferential surface and the second inner circumferential surface; a center electrode having a head supported by the ledge and extending in a space surrounded by the second inner circumferential surface, and a circular columnar leg extending continuously from a forward end of the head in a space surrounded by the first inner circumferential surface; and a seal material charged into a space surrounded by the ledge, the second inner circumferential surface, and the head to thereby hold the center electrode in the axial bore; the spark plug being characterized in that: a position P1 on the center electrode is where an outside diameter of the center electrode begins to increase, beyond an average outside diameter H of the leg, from the leg toward a rear end of the center electrode, and when an axial distance E along the axis from the position P1 to a forward end of the center electrode is 15 mm or more, an axial distance A along the axis from the position P1 to the rear end of the center electrode is from 3.8 mm to 5.0 mm. a position P1 on the center electrode is where an outside diameter of the center electrode begins to increase, beyond an average outside diameter H of the leg, from the leg toward a rear end of the center electrode, and when an axial distance E along the axis from the position P1 to a forward end of the center electrode is 15 mm or more, an axial distance A along the axis from the position P1 to the rear end of the center electrode is from 3.8 mm to 5.0 mm.