Patent ID: 8791627

Claim:
A spark plug, comprising: an insulator having an axial hole penetrating in an axial direction and a center electrode disposed at a tip end side of the axial hole; and a tubular metal shell disposed at an outer periphery of the insulator for holding the insulator, the metal shell including a shoulder formed to project from an inner peripheral surface of the metal shell inward in a radial direction, wherein the insulator includes: a lock portion locked at the shoulder; a trunk portion formed at a tip end side of the lock portion; and a leg formed at the tip end side of the trunk portion, the leg including a reduced diameter portion with an outer diameter reduced toward the tip end side, the leg having a smaller outer diameter than an outer diameter of the trunk portion, wherein the shoulder of the metal shell includes: a first shoulder that has an inner diameter reduced from a rear end side toward the tip end side; and a second shoulder formed at the tip end side of the first shoulder, the second shoulder extending to face the trunk portion, wherein the tip end of the second shoulder is positioned at the tip end side with respect to the tip end of the trunk portion in the axial direction, a distance Da between the tip end of the second shoulder and the leg along the radial direction and a distance Db between the tip end of the trunk portion and the second shoulder along the radial direction satisfy a relationship in Expression 1, and a distance T and a distance L satisfy a relationship in Expression 2, the distance T being a distance between the rear end of the first shoulder and the tip end of the second shoulder along the axial direction, the distance L being a distance between the rear end of the first shoulder and a tip end face of the metal shell along the axial direction: Da/Db≧ 1.1 (Expression 1) T/L≦ 0.5 (Expression 2).