Patent ID: 8410674

Claim:
A spark plug comprising: a tubular metallic shell extending in a direction of an axis; a tubular insulator provided in the metallic shell, having a front end located frontward of a front end of the metallic shell in the direction of the axis, and having an axial hole extending in the direction of the axis; and a center electrode inserted into the axial hole and having a space-forming portion which forms, in cooperation with a wall surface of the axial hole, an annular space which is open frontward in the direction of the axis, and a main body portion extending rearward from a rear end of the space-forming portion and having a diameter greater than that of the space-forming portion, the spark plug being wherein the insulator has a thickness of 0.6 mm or less as measured on a cross section which is orthogonal to the axis and contains the front end of the metallic shell; and a distance, as measured along the axis, between a boundary portion and the front end of the metallic shell is set to 0.4 mm or greater, said boundary portion being formed between the space-forming portion and the main body portion.