There is provided a spark plug that includes a center electrode, an insulator, a housing, a ground electrode, an auxiliary ground electrode, and a projected portion in the housing. The housing has a though-hole which is formed in the housing along the longitudinal axis of the center electrode and has an opening of the through-hole at the first housing end. The ground electrode has a first end facing the end of the center electrode to form a first discharge gap. The auxiliary ground electrode has a first end facing an outer peripheral surface of the first end of the insulator to form a second discharge gap. The projected portion is formed in a hollow portion of the through-hole and is projected from an inner peripheral surface of the hollow portion of the housing.