A spark plug, comprised of: an insulator having an axial bore extending in the direction of an axis, a first inner circumferential surface extending at a forward side of the axial bore, a second inner circumferential surface extending at a rear side of the axial bore and having a diameter greater than that of the first inner circumferential surface, and a ledge connecting the first inner circumferential surface and the second inner circumferential surface; a center electrode having a head supported by the ledge and extending in a space surrounded by the second inner circumferential surface, and a circular columnar leg extending continuously from the forward end of the head in a space surrounded by the first inner circumferential surface; and a seal material for holding the center electrode in the axial bore.