PURPOSE: To prevent covering of an ignition plug so as to prevent misfire of an engine, engine stall, and defective starting.

CONSTITUTION: Right and left center electrodes 22 on the inside and right and left earth electrodes 23 on the outside are arranged centering around a nozzle 25 with same width on one straight line. The center electrodes 22 are standingly provided in parallel with the axis from a valve body 30, and formed into curved plates so as to be cut off at both sides of the concentric circle. Meanwhile, the earth electrodes 23 are standingly provided from a housing 31 in parallel with the axis, and the extreme side is bent inside into a nearly L-shape. The shape of the center electrode 22 is set so that the central angle (β) corresponding to the width of the inner circumferential face 22a of the center electrode 22, and the central angle (α) corresponding to the width of the outer circumferential face 22b are in the relation (α)<(β). Namely, the section of the center electrode 22 perpendicular to the axial direction is formed into a shape being cut off from a ring by a prescribed width from a ring.