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 of the lock portion; and a leg formed at a tip end of the trunk portion, the leg including a reduced diameter portion with an outer diameter reduced toward a 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 toward the tip end side; and a second shoulder formed at a tip end of the first shoulder, the second shoulder extending to face the trunk portion, wherein a 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 T / L ≤ 0.5 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 of the lock portion; and a leg formed at a tip end of the trunk portion, the leg including a reduced diameter portion with an outer diameter reduced toward a tip end side, the leg having a smaller outer diameter than an outer diameter of the trunk portion, wherein a lock portion locked at the shoulder; a trunk portion formed at a tip end of the lock portion; and a leg formed at a tip end of the trunk portion, the leg including a reduced diameter portion with an outer diameter reduced toward a 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 toward the tip end side; and a second shoulder formed at a tip end of the first shoulder, the second shoulder extending to face the trunk portion, wherein a 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 T / L ≤ 0.5 a first shoulder that has an inner diameter reduced from a rear end toward the tip end side; and a second shoulder formed at a tip end of the first shoulder, the second shoulder extending to face the trunk portion, wherein a 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 T / L ≤ 0.5