Patent ID: 8188641

Claim:
A spark plug comprising: a center electrode; an insulating body which has an axial hole extending in an axis line direction and holds the center electrode in the axial hole; a metal shell which holds the insulating body and surrounds a radial periphery of the insulating body; and a ground electrode having one end portion joined to the metal shell, the ground electrode being bent such that an other end portion opposes a front end portion of the center electrode so as to form a spark gap between the other end portion and the center electrode, wherein the center electrode comprises: an electrode base member comprising an electrode base member mount tapered in a front end portion thereof such that a diameter thereof is reduced toward a front end side; and a noble metal tip joined to a front end surface of the electrode base member mount, wherein the noble metal tip and the electrode base member are joined via a fused portion in which a component of the noble metal tip and a component of the electrode base member mount are fused, and CL>0.5 mm is satisfied where CL is a shortest distance in the axis line direction from a tip side boundary defined between the fused portion and the noble metal tip to a front end of the noble metal tip, and wherein S 1 <S 2 and 5 mm≦( S 3 −S 2)/ D ≦50 mm are satisfied where S 1 is a cross section area of the tip side boundary, S 2 is a cross section area of a base member side boundary defined between the fused portion and the electrode base member mount, S 3 is a cross section area of a base portion of the electrode base member mount, and D is a shortest distance in the axis line direction from the base member side boundary to the base portion, wherein the center electrode is held by the insulating body in a state where at least the noble metal tip, the fused portion, and the electrode base member mount are exposed from the insulating body, wherein the ground electrode is bent such that the other end portion of the ground electrode opposes a side surface of the noble metal tip, and the spark gap GA is formed between the other end portion of the ground electrode and the side surface of the noble metal tip, and wherein GB/GA >1.05 is satisfied where GB is a shortest distance from the other end portion of the ground electrode to the tip side boundary.