Spark plug

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 core includes a first core member located on an inner side thereof and a second core member located on an outer side thereof so as to circumferentially surround the first core member and having a higher thermal conductivity and a lower hardness than those of the first core member;
• wherein the first core member has a protruding shape that protrudes to a rear of the second core member in the axis direction;
• wherein the weld region has an undulating shape that undulates according to the protruding shape of the first core member;
• and
• wherein a distance between frontmost and rearmost ends of the weld region in the axis direction is 0.15 mm or more.