To attain stabilization of firing, improvement in flame propagation speed, improvement in output, purification of exhaust gas, stable firing of heterogeneous air-fuel mixture, improvement in fuel consumption or the like by enabling stable and high-efficiency combustion/reaction even in combustion/reaction of a lean air-fuel mixture with a reduced ratio of fuel to the air-fuel mixture in a heat engine such as a reciprocal engine, a rotary engine, a jet engine or a gas turbine or a plasma device.

The ignition device comprises a dielectric constant control means introducing water and/or exhaust gas to a combustion/reaction chamber 8 to control the dielectric constant of air-fuel mixture in the combustion/reaction chamber 8, a microwave emitting antenna 1 emitting microwaves into the chamber 8, and a discharge part 2 firing the air-fuel mixture in the chamber 8. The dielectric constant control means controls the dielectric constant of the air-fuel mixture prior to combustion/reaction of the air-fuel mixture, and matches the resonance frequency of the air-fuel mixture with the frequency of the microwaves.

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