A spark plug (1, 1a), comprising: an insulator having an axial hole extending in a direction of an axial line; a central electrode held in the axial hole at a portion closer to a first end of the axial hole; a metal terminal held in the axial hole at a portion closer to a second end of the axial hole; an electrically-conductive seal portion that occupies the axial hole of the insulator to electrically connect the central electrode and the metal terminal to each other; and a metal shell that houses the insulator, wherein the electrically-conductive seal portion includes a large-diameter seal portion, a small-diameter seal portion disposed closer to a tip end of the spark plug than the large-diameter seal portion and having an outer diameter smaller than an outer diameter of the large-diameter seal portion, and a tapered seal portion disposed between the large-diameter seal portion and the small-diameter seal portion, wherein the insulator includes a flange portion, a first middle-body portion disposed closer to the tip end than the flange portion and having an outer diameter smaller than an outer diameter of the flange portion, and a leg portion disposed closer to the tip end than the first middle-body portion and having an outer diameter that tapers toward the tip end; wherein the tapered seal portion is disposed inside the first middle-body portion; characterized in that: the leg portion extends towards the tip end of the spark plug from a packing interposed between the insulator and an inner circumferential portion of the metal shell; the insulator further includes: a second middle-body portion disposed closer to the tip end than the first middle-body portion and between the first middle-body portion and the leg portion and having an outer diameter smaller than an outer diameter of the first middle-body portion, and a tapered portion disposed closer to the tip end than the second middle-body portion and having an outer diameter that tapers towards the tip end; the leg portion is disposed closer to the tip end than the tapered portion; and the packing is interposed between the tapered portion and the inner circumferential portion of the metal shell. an insulator having an axial hole extending in a direction of an axial line; a central electrode held in the axial hole at a portion closer to a first end of the axial hole; a metal terminal held in the axial hole at a portion closer to a second end of the axial hole; an electrically-conductive seal portion that occupies the axial hole of the insulator to electrically connect the central electrode and the metal terminal to each other; and a metal shell that houses the insulator, wherein the electrically-conductive seal portion includes a large-diameter seal portion, a small-diameter seal portion disposed closer to a tip end of the spark plug than the large-diameter seal portion and having an outer diameter smaller than an outer diameter of the large-diameter seal portion, and a tapered seal portion disposed between the large-diameter seal portion and the small-diameter seal portion, wherein the insulator includes a flange portion, a first middle-body portion disposed closer to the tip end than the flange portion and having an outer diameter smaller than an outer diameter of the flange portion, and a leg portion disposed closer to the tip end than the first middle-body portion and having an outer diameter that tapers toward the tip end; wherein the tapered seal portion is disposed inside the first middle-body portion; characterized in that: the leg portion extends towards the tip end of the spark plug from a packing interposed between the insulator and an inner circumferential portion of the metal shell; the insulator further includes: a second middle-body portion disposed closer to the tip end than the first middle-body portion and between the first middle-body portion and the leg portion and having an outer diameter smaller than an outer diameter of the first middle-body portion, and a tapered portion disposed closer to the tip end than the second middle-body portion and having an outer diameter that tapers towards the tip end; the leg portion is disposed closer to the tip end than the tapered portion; and the packing is interposed between the tapered portion and the inner circumferential portion of the metal shell. the leg portion extends towards the tip end of the spark plug from a packing interposed between the insulator and an inner circumferential portion of the metal shell; the insulator further includes: a second middle-body portion disposed closer to the tip end than the first middle-body portion and between the first middle-body portion and the leg portion and having an outer diameter smaller than an outer diameter of the first middle-body portion, and a tapered portion disposed closer to the tip end than the second middle-body portion and having an outer diameter that tapers towards the tip end; the leg portion is disposed closer to the tip end than the tapered portion; and the packing is interposed between the tapered portion and the inner circumferential portion of the metal shell.