A spark plug has a metal shell and a ground electrode joined at a base end portion thereof to the metal shell and includes a surface layer and a core higher in thermal conductivity than the surface layer. The surface layer has a thickness of 0.2 to 0.4 mm at a specific position that is located 1 mm from the base end portion. 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.