Spark plug

A spark plug comprising:

• an insulator having an axial hole;
• a metal shell surrounding an outer circumference of the insulator;
• a center electrode having a center electrode base material disposed in the axial hole and an electrode tip joined to the center electrode base material and exposed outside from a front end portion of the metal shell;
• and
• a ground electrode having a fixed end portion fixed to the metal shell and a free end portion located at a predetermined gap apart from a front end of the electrode tip, the ground electrode having:
• an inner surface facing the center electrode and the insulator;
• and an outer surface connecting one end to the other end of the inner surface in a width direction of the ground electrode and including a back surface located opposite the inner surface, the ground electrode having a center electrode-facing site opposed to and facing the center electrode,
• wherein the spark plug further comprises a coating part formed of noble metal or noble metal alloy such that the coating part covers at least a region of the inner surface from a first intersection to a second intersection without covering the back surface, where the first intersection is defined as containing an intersection point at which an imaginary line extending from an outer circumference of the center electrode base material at a side of the fixed end portion to the ground electrode intersects the ground electrode;
• and the second intersection is defined as an intersection at which an imaginary plane passing through a midpoint of the predetermined gap and extending in parallel with an end face of the front end intersects the ground electrode;
• wherein the spark plug satisfies a relationship of 0.7 F≦A≦B where A is a dimension of the coating part in the width direction;
• B is a dimension of the ground electrode in the width direction;
• and F is a width of the front end of the electrode tip;
• and
• wherein, when the ground electrode, the coating part and the electrode tip are visually observed from a side of the free end portion, a center line of the coating part perpendicular to the width direction is in a range of the width of the electrode tip.