There is provided an ignition plug whereby an electrical configuration for discharge and an electrical configuration for guiding and radiating microwaves are provided concomitantly, within dimensions permitting installation in an installation space for an ordinary ignition plug, and adapted to efficiently guide and transmit microwaves into a combustion chamber. The ignition plug comprises a center electrode and a ground electrode for spark discharge; a center wire and an outside conductor forming a coaxial electromagnetic wave transmission line; and a microwave radiation antenna that is electrically integrated with the center wire; the microwave radiation antenna having a profile that defines part of a sphere or arc including a plurality of locations substantially equidistant from the center electrode.