Patent ID: 8922104

Claim:
A spark plug comprising: a tubular insulator having an axial hole extending therethrough in an axial direction; a center electrode inserted into a forward end portion of the axial hole; a tubular metallic shell provided around the insulator; a rodlike ground electrode whose proximal end portion is fixed to a forward end portion of the metallic shell; and a tip that is joined to the ground electrode such that at least a portion of the tip along a thickness direction of the tip is embedded in an inner side surface of the ground electrode facing the center electrode, wherein a gap is formed between the tip and a forward end portion of the center electrode, the tip is joined to the ground electrode through a fusion zone which is provided between the ground electrode and a proximal side of the tip facing the proximal end portion of the ground electrode, the tip and the ground electrode being fused at the fusion zone, and on a section which contains a center axis of the ground electrode and is in parallel with the thickness direction of the tip, a relational expression E/F≧1.1 is satisfied, where E (mm) is a longest distance along the thickness direction from the inner side surface to a boundary between the fusion zone and the ground electrode, and F (mm) is a largest amount of embedment of the tip in the inner side surface along the thickness direction.