Patent ID: 8629605

Claim:
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 frontward direction is defined as a direction parallel to the axial direction toward a spark portion formed between the center electrode and the ground electrode, and a direction opposite to the frontward direction is defined 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, wherein 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, and B is a thickness of a most thick-walled subportion having a largest radial wall thickness of the diameter increase portion.