An erosion of an electrode is prevented in a spark plug that performs an ignition by use of a spark discharge and an electromagnetic wave. The spark plug that a pulse voltage for a spark discharge and an electromagnetic wave supplied into the spark discharge as an energy are applied to a center electrode, the spark plug comprises the center electrode, an insulator including an axis hole into which the center electrode is engaged, a ground electrode configured to form a space between the ground electrode and the center electrode in which the spark discharge is generated, and a cylindrical conductor that is arranged at an outer circumference of the insulator, and an annular member is formed at an inner circumference of the cylindrical conductor and at a position where a distance from a distal end surface of the cylindrical conductor is within a range between ⅛ wavelength and ¼ wavelength of the electromagnetic wave.