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 side surface of the center electrode; and a noble metal tip provided on a surface of the ground electrode which faces the side surface of the center electrode, and forming a spark discharge gap in cooperation with the center electrode; the spark plug being characterized in that: a fusion zone is formed at 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; the thickness D x of the fusion zone as measured along a longitudinal direction of the ground electrode increases frontward with respect to the axial direction; and when D represents the thickness of the thickest portion of the fusion zone as measured along the longitudinal direction of the ground electrode, and E represents the length of the longest portion of the fusion zone as measured along the axial direction, a relation 1.5 ≤ E/D is satisfied. 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 side surface of the center electrode; and a noble metal tip provided on a surface of the ground electrode which faces the side surface of the center electrode, and forming a spark discharge gap in cooperation with the center electrode; the spark plug being characterized in that: a fusion zone is formed at 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; the thickness D x of the fusion zone as measured along a longitudinal direction of the ground electrode increases frontward with respect to the axial direction; and when D represents the thickness of the thickest portion of the fusion zone as measured along the longitudinal direction of the ground electrode, and E represents the length of the longest portion of the fusion zone as measured along the axial direction, a relation 1.5 ≤ E/D is satisfied. a fusion zone is formed at 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; the thickness D x of the fusion zone as measured along a longitudinal direction of the ground electrode increases frontward with respect to the axial direction; and when D represents the thickness of the thickest portion of the fusion zone as measured along the longitudinal direction of the ground electrode, and E represents the length of the longest portion of the fusion zone as measured along the axial direction, a relation 1.5 ≤ E/D is satisfied.