Spark plug with specific gap between insulator and electrodes

A spark plug comprising:

• an insulator having a central through hole;
• a central electrode held in said central through hole and protruded from the lower end face of said insulator downward;
• a main metal shell for holding said insulator;
• a parallel ground electrode having one end which is bonded to the main metal shell and the other end which is arranged so that the other end is opposite to the end face of said central electrode for holding said insulator and a parallel ground electrode arranged, in which a main air gap is formed by said parallel ground electrode and the end face of said central electrode;
• and
• at least one semi-surface ground electrode having one end which is bonded to said main metal shell and the other end which is arranged so that the other end is opposite to the side face of said central electrode or the side face of said insulator, a semi-surface gap being formed between the end face of the other end of said semi-surface ground electrode and the side face of said central electrode opposite to the end face, a gap between a semi-surface and the insulator is formed between the end face of said semi-surface ground electrode and the side face of said insulator opposite to the end face;
• wherein difference in a level E between the height of the lower end face of said insulator and the height of the upper edge of the end face of said semi-surface ground electrode is equal to or smaller than +0.7 mm where ‘+’ means a direction in which the upper edge of the end face of the semi-surface ground electrode separates downward from the lower end face of the insulator;
• the distance B of said semi-surface gap (B) is longer than the distance A of said main air gap (A);
• and
• if a first extended line acquired by extending a line showing said lower end face of said insulator outward, a second extended line acquired by extending a line showing the side face in the vicinity of the semi-surface gap (B) of said insulator in the direction of said lower end face and a third extended line (33) acquired by extending a line showing the end face of said semi-surface ground electrode downward are drawn in case the end face of said semi-surface ground electrode and said insulator are cut along the central axis of said insulator, distance that is a distance C of a semi-surface air gap (C) between the intersection of said first and second extended lines and the intersection (P2) of said first and third extended lines is shorter than the distance A of said main air gap (A).