Spark plug

A spark plug (100) comprising an insulator (1) having a center through-hole (1C) formed therein;

• a center electrode (2) held in the center through-hole, disposed in a tip portion (1D) of said insulator, and having a noble metal chip (105) located at a tip portion thereof;
• an electrode base material (2n) which forms a surface layer portion of the center electrode;
• a metallic shell (5) for holding said insulator such that a tip portion of said insulator projects from a tip face (5D) thereof;
• a parallel ground electrode (11) disposed such that one end thereof is joined to the tip face of said metallic shell while the other end thereof faces a tip face of said center electrode so as to form a main air gap (α);
• and a plurality of semi-creepage ground electrodes (12) each disposed such that one end thereof is joined to said metallic shell while the other end thereof faces at least either a side peripheral surface of said center electrode or a side peripheral surface of said insulator so as to form a semi-creepage gap (β),
• said spark plug being characterized in that a tip portion of said center electrode (2) as projected orthogonally on a virtual plane in parallel with an axis of said center electrode includes a tapered portion which is tapered such that the diameter thereof reduces toward the tip face of the center electrode in the axial direction;
• a convex portion (2k) having a convex vertex (P) is formed at an axially intermediate position of the tapered portion such that an outline thereof as viewed on the virtual plane projects radially outward with respect to the axis;
• an axially measured distance between a convex vertex of the convex portion and a tip of said insulator is less than 0.5 mm;
• a heat release acceleration metal portion (2m), higher in thermal conductivity and linear expansion coefficient than the electrode base material, is present at a position located 1.5 mm axially rearward from the convex vertex while being enclosed by the electrode base material;
• and the electrode base material has a wall thickness (W) of not less than 0.6 mm measured at a position located 1.5 mm axially rearward from the convex vertex.