Patent ID: 8884503

Claim:
A spark plug, comprising: a rod-shaped center electrode extending in an axis direction of the spark plug; an insulator having an axial hole formed in the axis direction so as to retain the center electrode in the axial hole; a metal shell circumferentially surrounding and retaining the insulator; and a ground electrode having a base end portion welded to the metal shell and a distal end portion facing an axially front end portion of the center electrode with a gap left between the distal end portion of the ground electrode and the front end portion of the center electrode, the ground electrode including a surface layer defining a surface of the ground electrode and a core located inside the surface layer and having a higher thermal conductivity than that of the surface layer, the surface layer having a thickness of 0.2 mm to 0.4 mm at a specific position that is located 1 mm from the base end portion in a direction toward the distal end portion along an outer shape of the ground electrode, wherein the spark plug satisfies the following condition: W1≧W2×1.55−(W3+0.25) where W1 (mm) is a width of the metal shell at a weld region of the metal shell joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode; W2 (mm) is a thickness of the ground electrode at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer at the specific position in the specific direction.