To apply an alternating electric field of megahertz to combustion chamber gas of an internal combustion engine, thus improving ignition performance.

An ignition device is structured so as to detect charged body current running through the combustion chamber gas of the internal combustion engine by applying an electric field to the combustion chamber gas using a transformer for ignition and a core electrode of an ignition plug. The core electrode 31 of the ignition plug is connected with only a secondary winding 27 of the transformer for ignition to reduce floating electrostatic capacity between a connection part and the ground. To the electric field to be applied to the combustion chamber gas, the alternating field of megahertz of large electric energy is applied.