A plasma jet spark plug comprising: a center electrode; an insulator (10, 110, 210, 310, 410) having an axial bore extending in an axial direction, and retaining the center electrode within the axial bore in such a manner as to accommodate a front end face of the center electrode within a front end portion of the axial bore; a cavity formed in a front end portion (16, 116, 216, 316, 416) of the insulator (10, 110, 210, 310, 410), in a form of a recess defined by a wall surface of the axial bore and a front end face of the center electrode; a metallic shell (50, 250, 350, 351, 450, 451, 452, 453) surrounding and retaining the insulator (310, 410) from outside with respect to a radial direction perpendicular to the axial direction; and a ground electrode (30, 230, 330, 372, 430, 472, 473) disposed frontward of the front end portion (16, 116, 216, 316, 416) of the insulator (10, 110, 210, 310, 410) with respect to the axial direction and having a contact portion (38, 123, 124, 125, 126, 220, 320, 321, 420, 421, 422) contacting the front end portion (16, 116, 216, 316, 416) of the insulator (10, 110, 210, 310, 410) in an annular contact zone such that, as viewed from the axial direction, an opening of the cavity is located internally of the contact portion, and a communication section (331, 341, 431) for establishing communication between the cavity and an ambient atmosphere; characterized in that a clearance is formed between the insulator (310, 410) and the metallic shell (50, 250, 350, 351, 450, 451, 452, 453); wherein the ground electrode (330, 372, 430, 472, 473) is not in contact with the metallic shell (50, 250, 350, 351, 450, 451, 452, 453) with respect to the axial direction and is in contact with the metallic shell (50, 250, 350, 351, 450, 451, 452, 453) with respect to a radial direction perpendicular to the axial direction and is electrically connected with the metallic shell (50, 250, 350, 351, 450, 451, 452, 453) by means of an outer peripheral portion thereof being joined to the metallic shell (50, 250, 350, 351, 450,451,452,453); wherein the contact portion (38, 123, 124, 125, 126, 220, 320, 321, 420, 421, 422) of the ground electrode (330, 372, 430, 472, 473) closes the clearance between the insulator (10, 110, 210, 310, 410) and the metallic shell (50, 250, 350, 351, 450, 451,452,453). a center electrode; an insulator (10, 110, 210, 310, 410) having an axial bore extending in an axial direction, and retaining the center electrode within the axial bore in such a manner as to accommodate a front end face of the center electrode within a front end portion of the axial bore; a cavity formed in a front end portion (16, 116, 216, 316, 416) of the insulator (10, 110, 210, 310, 410), in a form of a recess defined by a wall surface of the axial bore and a front end face of the center electrode; a metallic shell (50, 250, 350, 351, 450, 451, 452, 453) surrounding and retaining the insulator (310, 410) from outside with respect to a radial direction perpendicular to the axial direction; and a ground electrode (30, 230, 330, 372, 430, 472, 473) disposed frontward of the front end portion (16, 116, 216, 316, 416) of the insulator (10, 110, 210, 310, 410) with respect to the axial direction and having a contact portion (38, 123, 124, 125, 126, 220, 320, 321, 420, 421, 422) contacting the front end portion (16, 116, 216, 316, 416) of the insulator (10, 110, 210, 310, 410) in an annular contact zone such that, as viewed from the axial direction, an opening of the cavity is located internally of the contact portion, and a communication section (331, 341, 431) for establishing communication between the cavity and an ambient atmosphere; characterized in that a clearance is formed between the insulator (310, 410) and the metallic shell (50, 250, 350, 351, 450, 451, 452, 453); wherein the ground electrode (330, 372, 430, 472, 473) is not in contact with the metallic shell (50, 250, 350, 351, 450, 451, 452, 453) with respect to the axial direction and is in contact with the metallic shell (50, 250, 350, 351, 450, 451, 452, 453) with respect to a radial direction perpendicular to the axial direction and is electrically connected with the metallic shell (50, 250, 350, 351, 450, 451, 452, 453) by means of an outer peripheral portion thereof being joined to the metallic shell (50, 250, 350, 351, 450,451,452,453); wherein the contact portion (38, 123, 124, 125, 126, 220, 320, 321, 420, 421, 422) of the ground electrode (330, 372, 430, 472, 473) closes the clearance between the insulator (10, 110, 210, 310, 410) and the metallic shell (50, 250, 350, 351, 450, 451,452,453).