Spark plug for internal combustion engines

A spark plug for internal combustion engines, comprising:

• a housing having a cylindrical shape;
• an insulator held inside the housing and having a cylindrical shape;
• a center electrode held inside the insulator;
• and
• an earth electrode having a gap-forming surface a portion of which forms a discharge gap between said portion and the tip surface of the center electrode, wherein
• the insulator includes an insulator protrusion protruding on a tip side of the housing,
• an outer peripheral surface of the insulator protrusion includes an insulator inclined surface extending inward toward a tip in a plug axial direction, in a straight line or a curve that is convex inward, in an axial parallel cross-section that is at least one of cross-sections passing through a plug center axis and parallel to the plug axial direction,
• in the axial parallel cross-section, a virtual straight line passing through both ends of the insulator inclined surface passes through the gap-forming surface,
• the housing includes a housing exposed portion which is exposed to a combustion chamber, an outer peripheral surface of the housing exposed portion includes a housing inclined surface inclined inward toward the tip in the plug axial direction, and in the axial parallel cross-section, a housing-side virtual straight line passing through both ends of the housing inclined surface passes through the gap-forming surface,
• with the proviso that a spark plug comprising a center electrode covered with a conductor cylinder inside an insulator is excluded.