Spark plug for internal combustion engine

A spark plug for an internal-combustion engine, comprising:

• a rod-like center electrode extending in an axis direction;
• an insulator having an axial bore in the axis direction in which the center electrode is inserted;
• a generally cylindrical metal shell provided on an outer circumference of the insulator;
• a ground electrode extending from a front end portion of the metal shell and disposed so that a front end thereof is bent toward the center electrode;
• a noble metal portion provided on the front end of the ground electrode which faces a front edge of the center electrode;
• and
• a gap formed between the ground electrode and the center electrode,
• wherein an entire front end portion of the ground electrode including the bent portion thereof is positioned outside of a virtual outer circumferential face that is formed by extending a front end outer circumferential face of the center electrode in the axis direction, and a terminating face of the ground electrode is positioned on a front end side in the axis direction with respect to a virtual face formed by extending a front end face of the center electrode, and
• wherein the following equation is satisfied:
• 1.1≦b/a≦1.6,
• where “a” (mm) represents a first minimal distance between the front end portion of the center electrode and a point on the noble metal portion closest to the center electrode, and
• where “b” (mm) represents a second minimal distance between the front end portion of the insulator and the same point on the noble metal chip.