Spark plug for internal combustion engine

A spark plug for an internal combustion engine comprising:

• a cylindrical housing having an axial direction;
• a cylindrical insulator that is held inside the housing;
• a center electrode that is held inside the insulator so that a tip portion projects outwards;
• a ground electrode that projects from a tip portion of the housing towards a tip side of the housing along the axial direction and forms a spark discharge gap between the ground electrode and the center electrode;
• and
• a tip projecting portion that projects from the tip portion of the housing towards the tip side, at a position differing from that of the ground electrode, wherein
• the tip projecting portion has a flat air guiding surface that faces the ground electrode side in a plug circumferential direction, and
• when viewed from a plug axial direction, when a straight line that connects the center, in the plug circumferential direction, of the erect portion of the ground electrode standing erect from the housing and a center point of the center electrode is a straight line, an extension line of the air guiding surface is a straight line, a distance between an intersection, between the straight line L and the straight line, and the center point of the center electrode is a, an angle formed by the straight line and the straight line M is b, a diameter of the housing is D, and the distance a is positive towards the side receding from the erect portion of the ground electrode and negative towards the side approaching the erect portion, all of expression to expression below are satisfied:
• b≧−67.8×(a/D)+27.4 (1)
• b≦−123.7×(a/D)+64.5 (2)
• −0.4≦(a/D)≦0.4 (3)
• 0°<b≦90° (4).