Abstract: A spark plug comprises: a spark plug housing; an axial core electrode disposed in a longitudinal direction in the spark plug housing; and a ground electrode having a terminal part spaced apart along the longitudinal direction from a distal end of the axial core electrode and disposed in a perpendicular relation with the axial core electrode, the terminal part having a circular through hole formed along an axis coaxial with the axial core electrode, the circular through hole having a tapered inner surface and including a plurality of spiral protrusions disposed on the tapered inner surface to induce a turbulent air flow in the combustion inflow.

Abstract: An improved spark plug (1) is disclosed, which comprises a plurality of spiral protrusions (7) which are protruded from an inner side of the calcular part in the direction of the center circular hole (5) so as to form a turbulent flow in the mixed gas when the gas compressed during a compression stroke is inputted into the spark forming space through the center circular hole (5) in a conventional spark plug which includes an axial core electrode (2) for a discharge spark, and a side surface ground electrode (4) which is opposite to the axial core electrode (2) with respect to a spark forming space with a circular part (6) having a center circular hole (5) being formed at a discharge end of the side surface round electrode.