A spark plug comprising: a center electrode extending in an axial direction; an insulator disposed externally of an outer circumference of the center electrode; a metallic shell disposed externally of an outer circumference of the insulator and having a ledge projecting with a predetermined width toward the insulator; and a ground electrode joined to the metallic shell; wherein, when a direction parallel to the axial direction directed toward a spark portion formed between the center electrode and the ground electrode is taken as a frontward direction, and an opposite direction is taken as a rearward direction, the insulator has a support portion which faces a rear stepped portion of the ledge and through which the insulator is supported, and the insulator further has, in a region which faces the ledge: a diameter reduction portion (70, 70b) whose outside diameter reduces along the frontward direction from the support portion, and a diameter increase portion (71, 71b) which is located frontward of the diameter reduction portion (70, 70b) and whose outside diameter increases along the frontward direction, characterized in that the spark plug satisfies a relational expression 0.84 ≤ A / B ≤ 0.95, where, when a direction perpendicular to the axial direction is taken as a radial direction, A is a thickness of a most thin-walled subportion having a smallest radial wall thickness of the diameter reduction portion (70, 70b), and B is a thickness of a most thick-walled subportion having a largest radial wall thickness of the diameter increase portion (71, 71b). a center electrode extending in an axial direction; an insulator disposed externally of an outer circumference of the center electrode; a metallic shell disposed externally of an outer circumference of the insulator and having a ledge projecting with a predetermined width toward the insulator; and a ground electrode joined to the metallic shell; wherein, when a direction parallel to the axial direction directed toward a spark portion formed between the center electrode and the ground electrode is taken as a frontward direction, and an opposite direction is taken as a rearward direction, the insulator has a support portion which faces a rear stepped portion of the ledge and through which the insulator is supported, and the insulator further has, in a region which faces the ledge: a diameter reduction portion (70, 70b) whose outside diameter reduces along the frontward direction from the support portion, and a diameter increase portion (71, 71b) which is located frontward of the diameter reduction portion (70, 70b) and whose outside diameter increases along the frontward direction, characterized in that the spark plug satisfies a relational expression 0.84 ≤ A / B ≤ 0.95, where, when a direction perpendicular to the axial direction is taken as a radial direction, A is a thickness of a most thin-walled subportion having a smallest radial wall thickness of the diameter reduction portion (70, 70b), and B is a thickness of a most thick-walled subportion having a largest radial wall thickness of the diameter increase portion (71, 71b). a diameter reduction portion (70, 70b) whose outside diameter reduces along the frontward direction from the support portion, and a diameter increase portion (71, 71b) which is located frontward of the diameter reduction portion (70, 70b) and whose outside diameter increases along the frontward direction, characterized in that the spark plug satisfies a relational expression 0.84 ≤ A / B ≤ 0.95, where, when a direction perpendicular to the axial direction is taken as a radial direction, A is a thickness of a most thin-walled subportion having a smallest radial wall thickness of the diameter reduction portion (70, 70b), and B is a thickness of a most thick-walled subportion having a largest radial wall thickness of the diameter increase portion (71, 71b).