The origin of the pseudogap formed across the Fermi level has been studied in relation to the Fermi surface-Brillouin zone interaction and the sp-d hybridization effect by performing the LMTO-ASA (Linear Muffin-Tin Orbital-Atomic Sphere Approximation) band calculations for the three electron compounds: the γ-phase Cu 5 Zn 8 alloy, the free-electron-like Al 3 0 Mg 4 0 Zn 3 0 Frank-Kasper-type 1/1-1/1-1/1 approximant and the Al 6 8 Cu 7 Ru 1 7 Si 8 Mackay-Icosahedral-type 1/1-1/1-1/1 approximant. We revealed that, in the free-electron-like Al-Mg-Zn approximant, the Fermi surface-Brillouin zone interaction is solely responsible for the formation of the pseudogap at the Fermi level. In the case of the γ-phase and the Al-Cu-Ru-Si approximant, where d-states are involved across the Fermi level, we have demonstrated that the Fermi surface-Brillouin zone interaction is still strong enough to produce the pseudogap near the Fermi level but that its depth and width are substantially enhanced by the sp-d hybridization effect which splits the d-states into the bonding and antibonding states across the Fermi level.