The Fermi surface of the layered organic superconductor kappa_L_-(DMEDO-TSeF)_2_[Au(CN)_4_](THF) has been investigated, where DMEDO-TSeF is dimethyl (ethylenedioxy) tetraselenafulvalene. Band-structure calculations show that the fundamental Fermi surface is not circular, but elliptical with an eccentricity of 0.76, leading to a large orbital overlap in the extended Brillouin zone. This symmetry is lowered by a structural phase transition at 209 K at which domains form. The magnetic breakdown orbit is observed in the Shubnikov–de Haas oscillations under the low-field region, consistent with the small energy gap at the zone boundary. The observation of the Shubnikov–de Haas effect shows that the present compound has a clean electronic system despite the domain structures.
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