The cavity of a plasma jet ignition plug is configured in a stepped shape; i.e., composed of a constricted portion and a diameter-increased portion (65). The inner diameter (B) of the constricted portion is made smaller than the inner diameter (A) of the diameter-increased portion. Further, as measured along the direction of the axis O, the length (Y) of the constricted portion is made equal to or greater than the length (X) of the diameter-increased portion. This configuration suppresses a pressure loss produced when the generated plasma expands within the cavity. As a result, the energy of the plasma at the time of jetting is increased, and the igniting performance for an air-fuel mixture can be improved.