Patent ID: 8640666

Claim:
A spark plug for an internal combustion engine comprising: a rod-shaped center electrode extending in an axial direction; a substantially-cylindrical insulator that has an axial hole extending in the axial direction and in which the center electrode is provided in the axial hole; a substantially-cylindrical metal shell provided around an outer periphery of the insulator; a ground electrode that extends from a leading end portion of the metal shell and whose distal end is bent toward the center electrode; and a noble metal tip joined to a distal end portion of the ground electrode, wherein: a gap is formed between the leading end portion of the center electrode and the noble metal tip; the noble metal tip is joined such that a center axis of the noble metal tip is displaced from a center axis of the ground electrode toward a base-end side in the axial direction; the ground electrode comprises an outer layer made of a nickel alloy and an inner layer made of a material that exhibits thermal conductivity superior to that of the outer layer; a distal end of the inner layer is situated closer to the axial line than a base end of the ground electrode; and an overlap area between the inner layer and the noble metal tip occupies a ratio of 25% or more of the inner layer in a projective plane, the projective plane being defined by projecting, along the center axis of the ground electrode, a plane of the ground electrode viewed from a distal-end face side of the ground electrode on a cross section of the ground electrode in which a maximum cross-sectional area of the inner layer is achieved among cross sections orthogonal to the center axis of the ground electrode.