Plasma jet spark plug

A plasma jet spark plug, comprising:

• a cylindrical insulator having an axial bore extending therethrough in an axial direction;
• a rod-shaped central electrode inserted into the axial bore so that a front end surface of the central electrode is located further to the rear end side in the axial direction than the front end of the insulator;
• a cylindrical metal shell disposed around an outer peripheral surface of the insulator;
• a ground electrode installed at a front end portion of the metal shell;
• and
• a cavity portion that is a substantially cylindrical space formed by the inner peripheral surface of the axial bore and the front end surface of the central electrode and opened toward a front end side of the plasma jet spark plug,
• wherein a tapered section having a reduced diameter toward a front end side in the axial direction is formed at a front end portion of the central electrode, and
• wherein the axial bore includes:
• a sidewall section forming the cavity portion;
• and
• a diameter-enlarged section extending from a rear end of the sidewall section and formed at a location corresponding to the tapered section of the central electrode in the axial direction, the diameter-enlarged section having an enlarged diameter toward a rear end side in the axial direction along an outer peripheral surface of the tapered section of the central electrode.