A spark plug (100, 100E, 100F) comprising: an insulator including a reduced outer diameter portion having an outer diameter that decreases toward a front side (Df) in a direction of an axis, and a leg portion which is a portion on the front side (Df) relative to the reduced outer diameter portion, the insulator forming a through-hole extending in the direction of the axis; a center electrode, at least a portion of which is inserted in the through-hole on the front side (Df); a metallic shell (50, 50E, 50F) disposed around the insulator in a radial direction, the metallic shell (50, 50E, 50F) including a reduced inner diameter portion having an inner diameter that decreases toward the front side (Df), the metallic shell (50, 50E, 50F) forming an annular gap in cooperation with an outer peripheral surface of the leg portion; a ground electrode electrically connected to the metallic shell (50, 50E, 50F), and forming a gap (g) in cooperation with the center electrode; and a packing disposed between the reduced outer diameter portion of the insulator and the reduced inner diameter portion of the metallic shell (50, 50E, 50F), wherein in a case where a position, closest to the front side (Df), in a contact portion of the packing and the metallic shell (50, 50E, 50F) is regarded as a contact end position, a distance in the radial direction between the outer peripheral surface of the leg portion of the insulator and an inner peripheral surface (55i, 55Ei, 55Fi) of the metallic shell (50, 50E, 50F) is regarded as a gap distance, and a position at a rear end of a maximum portion of the annular gap, in which the gap distance is maximum, is regarded as a maximum end position, the metallic shell (50, 50E, 50F) includes an increased inner diameter portion (511, 511b) having an inner diameter that increases toward a rear side (Dfr) in the direction of the axis, on the front side (Df) relative to the contact end position, and the maximum end position is located on the rear side (Dfr) relative to an intermediate position at which a distance in the direction of the axis between the contact end position and the front end (57, 57E, 57F) of the metallic shell (50, 50E, 50F) is divided into two halves, characterized in that the gap distance (812, 812E, 812F) at a front end (57, 57E, 57F) of the metallic shell (50, 50E, 50F) is larger than a distance of the gap (g) between the center electrode and the ground electrode. an insulator including a reduced outer diameter portion having an outer diameter that decreases toward a front side (Df) in a direction of an axis, and a leg portion which is a portion on the front side (Df) relative to the reduced outer diameter portion, the insulator forming a through-hole extending in the direction of the axis; a center electrode, at least a portion of which is inserted in the through-hole on the front side (Df); a metallic shell (50, 50E, 50F) disposed around the insulator in a radial direction, the metallic shell (50, 50E, 50F) including a reduced inner diameter portion having an inner diameter that decreases toward the front side (Df), the metallic shell (50, 50E, 50F) forming an annular gap in cooperation with an outer peripheral surface of the leg portion; a ground electrode electrically connected to the metallic shell (50, 50E, 50F), and forming a gap (g) in cooperation with the center electrode; and a packing disposed between the reduced outer diameter portion of the insulator and the reduced inner diameter portion of the metallic shell (50, 50E, 50F), wherein in a case where a position, closest to the front side (Df), in a contact portion of the packing and the metallic shell (50, 50E, 50F) is regarded as a contact end position, a distance in the radial direction between the outer peripheral surface of the leg portion of the insulator and an inner peripheral surface (55i, 55Ei, 55Fi) of the metallic shell (50, 50E, 50F) is regarded as a gap distance, and a position at a rear end of a maximum portion of the annular gap, in which the gap distance is maximum, is regarded as a maximum end position, the metallic shell (50, 50E, 50F) includes an increased inner diameter portion (511, 511b) having an inner diameter that increases toward a rear side (Dfr) in the direction of the axis, on the front side (Df) relative to the contact end position, and the maximum end position is located on the rear side (Dfr) relative to an intermediate position at which a distance in the direction of the axis between the contact end position and the front end (57, 57E, 57F) of the metallic shell (50, 50E, 50F) is divided into two halves, characterized in that the gap distance (812, 812E, 812F) at a front end (57, 57E, 57F) of the metallic shell (50, 50E, 50F) is larger than a distance of the gap (g) between the center electrode and the ground electrode.