Spark plug

A spark plug comprising:

• a center electrode extending in an axial direction;
• a cylindrical insulator provided with an axial hole into which the center electrode is inserted;
• a cylindrical metal shell disposed on an outer periphery of the insulator;
• and
• a ground electrode having one end fixed to a leading end of the metal shell and another end having a facing surface that faces a leading end surface of the center electrode,
• wherein the spark plug has an annular space which is formed by an outer peripheral surface of the center electrode and an inner peripheral surface of the axial hole and is opened toward a leading end side in the axial direction,
• wherein, when C (mm) is a distance between the outer peripheral surface of the center electrode and the inner peripheral surface of the axial hole in an opening of the annular space along a direction orthogonal to the axial line, C≧2 mm is satisfied, and
• wherein, in a cross-section which includes the axial line and is orthogonal to the center axis of the ground electrode, a contour line of a lateral surface of the ground electrode, which is adjacent to the facing surface, has a curved shape which is convexed outward,
• wherein a contour line of the facing surface is linearly shaped in the cross-section, and
• wherein, when B (mm) is an outer diameter of the leading end surface of the center electrode, and D (mm) is a length of the contour line of the facing surface in the cross-section, D≦B is satisfied.