A spark plug comprising: an insulator having an axial bore extending in a direction of an axis (O), 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 (O) from the position P1 to a forward end of the center electrode is 15 mm or more, an axial distance A along the axis (O) 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 (O), 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 (O) from the position P1 to a forward end of the center electrode is 15 mm or more, an axial distance A along the axis (O) 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 (O) from the position P1 to a forward end of the center electrode is 15 mm or more, an axial distance A along the axis (O) from the position P1 to the rear end of the center electrode is from 3.8 mm to 5.0 mm. <EOS>