Spark plug for internal combustion engine

A spark plug for an internal combustion engine comprising:

• a hollow cylindrical housing which is attached to an internal combustion engine to have a tip thereof facing a combustion chamber;
• a cylindrical porcelain insulator which is retained inside the housing;
• a center electrode which is retained inside the porcelain insulator and protrudes from a front end of the porcelain insulator;
• and
• an annular ground electrode which is secured to a front end portion of the housing and has an inner peripheral surface facing an outer peripheral surface of the center electrode,
• wherein the housing has a shoulder which is formed on an inner peripheral surface of the housing and tapers toward a front end of the housing in an axial direction of the spark plug,
• wherein the porcelain insulator includes a mounting shoulder and an insulator nose, the mounting shoulder being formed on an outer periphery of the porcelain insulator and tapering toward a front end of the porcelain insulator in the axial direction of the spark plug, the mounting shoulder riding on the shoulder of the housing to retain the porcelain insulator inside the housing, the insulator nose being located closer to the front end of the porcelain insulator than the mounting shoulder is,
• wherein a pocket is created between an outer peripheral surface of the insulator nose and the inner peripheral surface of the housing,
• wherein a plurality of vent holes are formed in the front end portion of the housing and located outside the ground electrode in a radial direction of the housing, the vent holes communicating between the pocket and the combustion chamber, and
• wherein the vent holes include a first vent hole and a second vent hole, the first vent hole having an opening area which is different in size from an opening area of the second vent hole, as viewed in the axial direction of the spark plug.