Spark plug

A spark plug, comprising:

• an insulator having an axial hole penetrating in a direction of an axis;
• a center electrode inserted into the axial hole;
• a tubular metallic shell disposed at an outer circumference of the insulator;
• a ground electrode secured to a front end portion of the metallic shell, and bent to the axis side at a flexed portion;
• and
• a tip joined to a front end portion of the ground electrode to form a gap between the tip and a front end portion of the center electrode, wherein
• the tip is joined to the ground electrode with a part of the tip projecting from a front end face and an inner circumference-side side surface of the ground electrode,
• the ground electrode has a center of the front end face, the center being located at a front end side in the direction of the axis with respect to a front end of the center electrode;
• L/X≦1.28 is satisfied, where L (mm) represents a length of the ground electrode along a central axis of the ground electrode and X (mm) represents a projection length of the ground electrode relative to a front end face of the metallic shell along the axis, and
• 8.4-mm−1≦(S1/S2)/A is satisfied, where S1 (mm2) represents a cross section area of the ground electrode of a portion at a base end side of the ground electrode with respect to a portion where the tip is joined to the ground electrode in cross section perpendicular to the central axis of the ground electrode, S2 (mm2) represents a cross section area of the tip in cross section perpendicular to a projection direction of the tip relative to the front end of the ground electrode, and A (mm) represents a projection length of the tip relative to the front end face of the ground electrode in a longitudinal direction of the ground electrode.