A spark plug comprising: an insulator having an axial hole extending therethrough in an axial direction (OD); 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 (TD) of the ground electrode increases frontward with respect to the axial direction (OD); and when D represents the thickness of the thickest portion of the fusion zone as measured along the longitudinal direction (TD) of the ground electrode , and E represents the length of the longest portion of the fusion zone as measured along the axial direction (OD), a relation 1.5 ≤ E/D is satisfied. an insulator having an axial hole extending therethrough in an axial direction (OD); 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 (TD) of the ground electrode increases frontward with respect to the axial direction (OD); and when D represents the thickness of the thickest portion of the fusion zone as measured along the longitudinal direction (TD) of the ground electrode , and E represents the length of the longest portion of the fusion zone as measured along the axial direction (OD), 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 (TD) of the ground electrode increases frontward with respect to the axial direction (OD); and when D represents the thickness of the thickest portion of the fusion zone as measured along the longitudinal direction (TD) of the ground electrode , and E represents the length of the longest portion of the fusion zone as measured along the axial direction (OD), a relation 1.5 ≤ E/D is satisfied. <EOS>