In a spark plug in which one end of a ground electrode 9 is welded to a front end surface (7a) of a tubular metallic shell (7), the following relations (1) and (2) are satisfied: K≧1.1A  (1) K≧(D−d)/2  (2) where A represents the wall thickness of the metallic shell in the radial direction measured on the front end surface at a position where the wall thickness becomes the minimum; d represents the maximum inner diameter of the front end surface; D represents the minimum outer diameter of the front end surface; and K represents the wall thickness in a region of the front end surface where the ground electrode is welded to the front end surface.