A spark plug includes: a center electrode; an insulator having a through hole around a part of the center electrode; and a metal shell holding the insulator from an outer peripheral side thereof. The metal shell includes a shelf portion that projects radially inward. The insulator includes an engagement portion engaged with the shelf portion from the front side, and a front end portion at the front side with respect to a front end of the metal shell. The front end portion has an outer diameter larger than an inner diameter of the metal shell at the front side with respect to the shelf portion. The front end portion of the insulator has a diameter-enlarged portion at which a diameter of the through hole increases and which is spaced apart from an outer peripheral surface of the center electrode.