A spark plug (100; 200; 300) comprising: a tubular insulator having an axial through hole and a first insulator stepped portion that reduces in an outer diameter toward the front; a rod-shaped center electrode disposed in the through hole of the insulator; a metallic shell having a first metallic shell stepped portion that reduces in an inner diameter toward the front and supports the insulator through engagement of the first metallic shell stepped portion and the first insulator stepped portion by interposing therebetween a packing; and a ground electrode connected at an end to a front end surface of the metallic shell and facing at the other end portion toward the center electrode for thereby forming a spark discharge gap (g) between said other end portion and the center electrode, wherein the insulator includes a second insulator stepped portion that lies at a more front side than the first insulator stepped portion and reduces in diameter toward the front and an insulator intermediate diameter portion that lies between the first insulator stepped portion and the second insulator stepped portion; the metallic shell includes a second metallic shell stepped portion that lies at a more front end side than the first metallic shell stepped portion and increases in diameter toward the front and a small diameter portion that lies between the first metallic shell stepped portion and second metallic shell stepped portion; the insulator and the metallic shell, when observed in a section made by a plane including the axis of the spark plug, have therebetween a gap (D1-d1)/2 of less than 0.45 mm between the insulator intermediate diameter portion and the metallic shell smaller diameter portion; the gap (D1-d1)/2 is provided axially from a most front end side engagement position of the packing and the first insulator stepped portion as a starting point to a finishing point that is apart from the starting point by a length of 1.2 mm or more while being apart from the front end surface of the metallic shell by 7.9 mm or more toward a rear end side; characterized in that the rear end of the second insulator stepped portion is axially disposed at a more front end side than the front end of the first insulator stepped portion by an amount ranging from 1 to 6 mm. a tubular insulator having an axial through hole and a first insulator stepped portion that reduces in an outer diameter toward the front; a rod-shaped center electrode disposed in the through hole of the insulator; a metallic shell having a first metallic shell stepped portion that reduces in an inner diameter toward the front and supports the insulator through engagement of the first metallic shell stepped portion and the first insulator stepped portion by interposing therebetween a packing; and a ground electrode connected at an end to a front end surface of the metallic shell and facing at the other end portion toward the center electrode for thereby forming a spark discharge gap (g) between said other end portion and the center electrode, wherein the insulator includes a second insulator stepped portion that lies at a more front side than the first insulator stepped portion and reduces in diameter toward the front and an insulator intermediate diameter portion that lies between the first insulator stepped portion and the second insulator stepped portion; the metallic shell includes a second metallic shell stepped portion that lies at a more front end side than the first metallic shell stepped portion and increases in diameter toward the front and a small diameter portion that lies between the first metallic shell stepped portion and second metallic shell stepped portion; the insulator and the metallic shell, when observed in a section made by a plane including the axis of the spark plug, have therebetween a gap (D1-d1)/2 of less than 0.45 mm between the insulator intermediate diameter portion and the metallic shell smaller diameter portion; the gap (D1-d1)/2 is provided axially from a most front end side engagement position of the packing and the first insulator stepped portion as a starting point to a finishing point that is apart from the starting point by a length of 1.2 mm or more while being apart from the front end surface of the metallic shell by 7.9 mm or more toward a rear end side; characterized in that the rear end of the second insulator stepped portion is axially disposed at a more front end side than the front end of the first insulator stepped portion by an amount ranging from 1 to 6 mm.