A plasma jet ignition plug including an insulating body having an axial hole therethrough. A center electrode is inserted into the axial hole. A cavity portion is defined by the insulating body and center electrode, with the leading end of the axial hole as an opening end. A decreasing diameter portion decreasing in diameter toward a leading end side is formed on the axial hole, the leading end of the decreasing diameter portion is positioned closer to the leading end side than the leading end face of the center electrode. The inside diameter of the leading end of the decreasing diameter portion is made smaller than the outside diameter of the leading end face of the center electrode.