Patent ID: 8841827

Claim:
A spark plug comprising: an insulator having an axial bore extending therethrough in an axial direction; a center electrode provided in a forward end portion of the axial bore; a substantially tubular metallic shell which holds the insulator; a ground electrode whose one end is attached to a forward end portion of the metallic shell and whose other end faces a forward end portion of the center electrode; a noble metal tip provided at a position on the ground electrode which faces a forward end surface of the center electrode, and adapted to form a gap in cooperation with the forward end surface of the center electrode; and a fusion zone where the ground electrode and the noble metal tip are fused together formed in at least a portion of an interfacial region between the ground electrode and the noble metal tip, wherein when the fusion zone is projected in the axial direction, the projected fusion zone overlaps 70% or more of an area of the noble metal tip; and as viewed on a section which passes through a center of gravity of the noble metal tip and is perpendicular to a longitudinal direction of the ground electrode, the fusion zone has such a shape as to extend from a side surface of the ground electrode, a thickness of the fusion zone along the axial direction gradually reduces along a direction directed away from the side surface of the ground electrode, and a relational expression 1.3≦B/A is satisfied, where A is a greatest thickness of the fusion zone along the axial direction, and B is a length from a portion having the greatest thickness of the fusion zone to an inner end of the fusion zone.