A spark plug has a housing, an insulator, a central electrode and a ground electrode. At least one projection part is formed on an outer peripheral surface of the insulator at a location facing a distal end cylindrical surface of the insulator in a radial direction of the spark plug so that at least one projection part has a minimum distance measured from a plug central axis of the spark plug, which is longer than a distance of another part of the insulator measured from the plug central axis. On a cross section of the spark plug, at least one projection part is formed on the outer peripheral surface of the insulator in a direction between at least one projection part and the plug central axis which crosses to a direction between a rod-shaped part and the plug central axis.