Spark plug having fusion zone

A spark plug comprising:

• a center electrode extending in a direction of an axis;
• a tubular insulator having an axial bore into which the center electrode is inserted;
• 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 tip whose proximal end portion is joined to a forward end portion of the center electrode and whose distal end portion forms a gap in cooperation with a distal end portion of the ground electrode
• wherein,
• the tip is joined to the center electrode via a fusion zone which is formed through radiation of a laser beam or an electron beam toward a side surface of the center electrode and in which the tip and the center electrode are fused together,
• the fusion zone is located toward a side from which the laser beam or the electron beam is radiated, and includes an exposed surface exposed to an external environment, and
• in a section which contains the axis and passes through a center of the exposed surface,
• a relational expression C−B≧0.02 mm is satisfied, where
• C (mm) is a distance along the axis on a side surface of the tip between the fusion zone and a distal end of the tip, and
• B (mm) is a distance along the axis between a distal end surface of the tip and a portion of the fusion zone located closer to the axis than the side surface of the tip and located closest in the fusion zone to the distal end surface of the tip.