A spark plug comprising: a center electrode extending in an axial direction (OD) ; 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 (OD) 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 whose outside diameter reduces along the frontward direction from the support portion , and a diameter increase portion which is located frontward of the diameter reduction portion 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 (OD) is taken as a radial direction, A is a thickness of a most thin-walled subportion (P) having a smallest radial wall thickness of the diameter reduction portion , and B is a thickness of a most thick-walled subportion (Q1) having a largest radial wall thickness of the diameter increase portion . a center electrode extending in an axial direction (OD) ; 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 (OD) 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 whose outside diameter reduces along the frontward direction from the support portion , and a diameter increase portion which is located frontward of the diameter reduction portion 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 (OD) is taken as a radial direction, A is a thickness of a most thin-walled subportion (P) having a smallest radial wall thickness of the diameter reduction portion , and B is a thickness of a most thick-walled subportion (Q1) having a largest radial wall thickness of the diameter increase portion . a diameter reduction portion whose outside diameter reduces along the frontward direction from the support portion , and a diameter increase portion which is located frontward of the diameter reduction portion 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 (OD) is taken as a radial direction, A is a thickness of a most thin-walled subportion (P) having a smallest radial wall thickness of the diameter reduction portion , and B is a thickness of a most thick-walled subportion (Q1) having a largest radial wall thickness of the diameter increase portion . <EOS>