A device is described for igniting an air-fuel mixture in an internal combustion engine using a high-frequency electric power source having a coaxial waveguide structure (5) into which the high-frequency electric power may be coupled and whose one end protrudes into the individual combustion chamber of a cylinder of the internal combustion engine. The one end of the coaxial waveguide structure (5) is designed as an igniter (7a) in such a way that, when a voltage potential is applied, a field structure (22), protruding into the combustion chamber, and thus a free-standing plasma in the air-fuel mixture at the inner conductor (7, 7a), projecting from the waveguide structure, is generatable via an abrupt and/or smooth cross-section change (21) of the inner conductor (7) and/or the outer conductor (6).