Patent ID: 8624473

Claim:
A spark plug comprising: an insulator having an axial hole extending therethrough in an axial direction; a center electrode provided at a front end portion of the axial hole; a substantially tubular metallic shell which holds the insulator; a ground electrode whose one end is attached to a front end portion of the metallic shell and whose other end faces a front end portion of the center electrode; and a noble metal tip provided on a surface of the ground electrode which faces the front end portion of the center electrode, and which forms a spark discharge gap in cooperation with the center electrode; the spark plug being characterized in that: a fusion zone is formed along at least a portion of the boundary between the ground electrode and the noble metal tip through fusion of a portion of the ground electrode and a portion of the noble metal tip; and when A represents the thickness of the thickest portion of the fusion zone as measured along the axial direction, and B represents the length of the longest portion of the fusion zone as measured along a longitudinal direction of the ground electrode, a relation 1.5≦B/A is satisfied.