Patent ID: 8853929

Claim:
A plasma jet ignition plug comprising: an insulator having an axial bore extending in a direction of an axis; a center electrode inserted into the axial bore in such a manner that a forward end surface thereof is located rearward of a forward end of the insulator with respect to the direction of the axis; a metallic shell disposed externally of an outer circumference of the insulator; and a ground electrode fixed to a forward end portion of the metallic shell, said ground electrode located at a forward end portion of the plasma jet ignition plug; a cavity being defined by an inner circumferential surface of the axial bore and the forward end surface of the center electrode; wherein the axial bore comprises: a first straight portion having a fixed inside diameter and extending forward, with respect to the direction of the axis, from a forward end surface of the center electrode, a gradually reducing diameter portion whose diameter reduces forward with respect to the direction of the axis from a forward end of the first straight portion, and as viewed on a section which contains the axis, a relational expression α≧10 is satisfied, where α (°) is an acute angle of less than 90° formed by a straight line orthogonal to the axis and an outline of the gradually reducing diameter portion.