The Fermi surface (FS) shape of the organic superconductor (MDT-TSF)(AuI2)0.436 with an incommensurate anion structure (MDT-TSF: methylenedithio-tetraselenafulvalene) has been investigated by means of the optical reflectance spectra and angular-dependent magnetoresistance oscillations (AMRO). The difference of the plasma frequencies between the interstack and the intrastack directions indicates that the overall band structure has small anisotropy (ωp,⊥/ω p,∥ ≈ 0.62). The observation of the AMRO shows the presence of a small closed orbit, which can be assigned to the overlapped area of the large FS. This demonstrates that the incommensurate anion potential is crucial to remove the degeneracy of the energy bands on the zone boundary. The magnetoresistance peak in the fields applied within the conducting layers shows an unusually large interlayer transfer integral ( t⊥ ≈ 1 meV) among organic superconductors.