IGNITION PLUG

To provide an ignition plug which is stably combusted.SOLUTION: An ignition plug comprises a cap part which is connected to a tip end portion of a main body fitting and covers end portions of a center electrode and a ground electrode from a front end side and in which multiple through holes are formed closer to a front end than the ground electrode. On a cross section including an axis, in at least a portion of a region between a first virtual straight line which passes a front end of the end portion of the ground electrode and is vertical to the axis and a second virtual straight line which passes rear ends of inner opening ends of the multiple through holes closest to the front end, an expanded portion of which the shortest distance from an outer surface to an inner surface of the cap part becomes longer from the rear end side to the front end side exists in the cap part. A cross-sectional area, of a region enclosed by the expanded portion, vertical to the axis becomes smaller from the rear end side to the front end side.SELECTED DRAWING: Figure 2 【課題】安定に燃焼できる点火プラグを提供すること。【解決手段】点火プラグは、主体金具の先端部に接続され、中心電極および接地電極の端部を先端側から覆うと共に、接地電極よりも先端側に複数の貫通孔が形成されたキャップ部を備え、軸線を含む断面において、キャップ部は、接地電極の端部の先端を通り軸線に垂直な第１仮想直線と、複数の貫通孔の最も先端側の内側開口端の後端を通り軸線に垂直な第２仮想直線と、の間の領域の少なくとも一部に、キャップ部の外面から内面までの最短距離が、後端側から先端側へ向かうにつれて大きくなる拡大部が存在し、拡大部に囲まれた領域の、軸線に垂直な断面積は、後端側から先端側へ向かうにつれて小さくなる。【選択図】図２