A front end portion of a center electrode 3 is a circular cylindrical
center-electrode noble-metal ablation resistance portion 31 whose radius r
mm and length l mm are determined so as to satisfy 5.ltoreq.l/r.sup.2
<20. An ignition-performance-improving ground electrode 4 is configured
such that a distal end portion thereof is bent in a direction toward the
center electrode 3; and a rear end-edge 32t of a distal end surface 4s,
32s is located frontward in relation to a front end surface 31a of the
center-electrode noble-metal ablation resistance portion 31. In orthogonal
projection on a projection plane P perpendicularly intersecting an axis O
of the center electrode 3, the rear end-edge 32t is located outward in
relation to the front end surface 31a of the center-electrode noble-metal
ablation resistance portion 31. A portion of the
ignition-performance-improving ground electrode 4 which includes at least
the rear end-edge 32t is a ground-electrode noble-metal ablation
resistance portion 32.
Μια μερίδα μπροστινών τελών ενός κεντρικού ηλεκτροδίου 3 είναι μια κυκλική κυλινδρική μερίδα 31 αντίστασης αφαήρεσης ευγενής-μετάλλων κέντρο-ηλεκτροδίων την της οποίας ακτίνα ρ χιλ. και μήκος λ χιλ. καθορίζεται ώστε να ικανοποιηθούν 5.ltoreq.l/$l*r.sup.2
