A spark plug includes a metal shell and a cap member joined to the metal shell to define a pre-chamber. The cap member includes an overlapping surface that overlaps the metal shell along the axial line direction, an inner facing surface positioned closer to the pre-chamber than is the overlapping surface, the inner facing surface facing the metal shell in the axial line direction, and an outer facing surface positioned closer to an outer periphery than is the overlapping surface, the outer facing surface facing the metal shell in the axial line direction. A portion of the cap member that is closer to the pre-chamber than is the overlapping surface s spaced from the metal shell. The metal shell and the cap member are joined together at at least one of the outer facing surface and the overlapping surface.