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EuGa 4 and EuAl 4 with the BaAl 4 -type tetragonal structure are Eu-divalent antiferromagnets with the Néel temperature T N = 16.5 K and 15.4 K, respectively. We clarified the Fermi surface properties from the results of de Haas-van Alphen experiments for EuGa 4 and EuAl 4 and energy band calculations of a non-4f reference compound SrGa 4 . Among two kinds of Fermi surfaces in EuGa 4 , there exists a cube-like electron Fermi surface with a vacant space in center, which most likely drives a charge density wave (CDW) via a nesting effect. This is realized in EuGa 4 under pressures, where the Fermi surface is expected to be modified so that the CDW transition is more easily induced. The similar CDW phenomenon is observed blow T CDW = 140 K in EuAl 4 at ambient pressure. Therefore, the cube-like Fermi surface below T CDW in EuAl 4 dose not exist at low temperatures because of the Fermi surface reconstruction, revealing only small ellipsoidal Fermi surfaces.

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