We carried out de Haas-van Alphen (dHvA) experiments for TaSi2, NbSi2, and VSi 2 with the hexagonal chiral structure, and compared them with the results of energy band calculations. The Fermi surface is found to be split into two different Fermi surfaces, refl ecting the non-centrosymmetric crystal structure. A magnitude of the antisymmetric spin-orbit interaction or a splitting energy between the two Fermi surfaces is determined to be 493K for dHvA branch α (α′) and 564K for branchβ (β′), where these dHvA branches correspond to main Fermi surfaces. The present splitting values are compared with 209 and 244K in NbSi2, and 19 and 39K in VSi2, respectively. The splitting energy is found to be larger in the Ta-5d conduction electrons than those with the Nb-4d and V-3d conduction electrons.