A spark plug for an internal combustion engine comprising: a rodlike center electrode extending in a direction of an axis; a tubular insulator provided externally of an outer circumference of the center electrode; a tubular metallic shell provided externally of an outer circumference of the insulator; a ground electrode disposed at a forward end portion of the metallic shell; and a noble metal tip formed from a noble metal alloy and provided at one object member of at least one of the center electrode and the ground electrode; the noble metal tip being joined to the object member via a fusion zone which contains components of a metal material used to form the object member, and components of the noble metal alloy used to form the noble metal tip, and the spark plug being characterized in that as viewed on a plane of projection which is orthogonal to a center axis of the noble metal tip and on which the noble metal tip and the fusion zone are projected along the center axis, a projected overlap region of the noble metal tip and the fusion zone accounts for 70% or more of a projected region of the noble metal tip, the metal material used to form the object member contains nickel as a main component and contains at least silicon out of aluminum and silicon such that the amount of silicon is 0.4% by mass or higher and such that the total amount of aluminum and silicon is in a specific mass percentage range; the fusion zone is formed of a plurality of fusion spots which are formed continuously through intermittent radiation of a laser beam or an electron beam; multi-fusion spots, each formed through overlap of three or more fusion spots, exist on a surface of the fusion zone; and as viewed along a baseline which passes through centers of the fusion spots on surfaces of the fusion spots, segments of the baseline which pass through the respective multi-fusion spots have a total length in a specific range with respect to a length of the baseline, wherein the specific mass percentage range of the total amount of aluminum and silicon is 0.5% by mass to 1.6% and the specific range of the total length of segments of the baseline is 35% or less of a length of the base line; or the specific mass percentage range is 0.5% to 1.9% and the specific range of the total length is 30% or less; or the specific mass percentage range is 0.5% to 5.0% and the specific range of the total length is 20% or less. a rodlike center electrode extending in a direction of an axis; a tubular insulator provided externally of an outer circumference of the center electrode; a tubular metallic shell provided externally of an outer circumference of the insulator; a ground electrode disposed at a forward end portion of the metallic shell; and a noble metal tip formed from a noble metal alloy and provided at one object member of at least one of the center electrode and the ground electrode; the noble metal tip being joined to the object member via a fusion zone which contains components of a metal material used to form the object member, and components of the noble metal alloy used to form the noble metal tip, and the spark plug being characterized in that as viewed on a plane of projection which is orthogonal to a center axis of the noble metal tip and on which the noble metal tip and the fusion zone are projected along the center axis, a projected overlap region of the noble metal tip and the fusion zone accounts for 70% or more of a projected region of the noble metal tip, the metal material used to form the object member contains nickel as a main component and contains at least silicon out of aluminum and silicon such that the amount of silicon is 0.4% by mass or higher and such that the total amount of aluminum and silicon is in a specific mass percentage range; the fusion zone is formed of a plurality of fusion spots which are formed continuously through intermittent radiation of a laser beam or an electron beam; multi-fusion spots, each formed through overlap of three or more fusion spots, exist on a surface of the fusion zone; and as viewed along a baseline which passes through centers of the fusion spots on surfaces of the fusion spots, segments of the baseline which pass through the respective multi-fusion spots have a total length in a specific range with respect to a length of the baseline, wherein the specific mass percentage range of the total amount of aluminum and silicon is 0.5% by mass to 1.6% and the specific range of the total length of segments of the baseline is 35% or less of a length of the base line; or the specific mass percentage range is 0.5% to 1.9% and the specific range of the total length is 30% or less; or the specific mass percentage range is 0.5% to 5.0% and the specific range of the total length is 20% or less.