A spark plug includes a tubular metallic shell; a tubular insulator in the tubular metallic shell located at a front end of the metallic shell and having an axial hole extending in the axial direction; and a center electrode inserted into the axial hole. The center electrode has a space-forming portion which forms, in cooperation with a wall surface of the axial hole, an annular space open frontward in the axial direction, and a main body portion extending rearward from the rear end of the space-forming portion. The thickness of the insulator is 0.6 mm or less as measured on a cross section and contains the front end of the metallic shell. The axial distance between the boundary portion and the front end of the metallic shell is 0.4 mm or greater, where the boundary portion is formed between the space-forming portion and the main body portion.