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; and 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; the spark plug being characterized in that: a fusion zone where the ground electrode and the noble metal tip are fused together is formed in at least a portion of an interfacial region between the ground electrode and the noble metal tip; 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 a shape and extends 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. 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; and 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; the spark plug being characterized in that: a fusion zone where the ground electrode and the noble metal tip are fused together is formed in at least a portion of an interfacial region between the ground electrode and the noble metal tip; 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 a shape and extends 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. a fusion zone where the ground electrode and the noble metal tip are fused together is formed in at least a portion of an interfacial region between the ground electrode and the noble metal tip; 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 a shape and extends 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.