Electronic structures of nine cubic perovskite-type oxides AB O3's including SrTiO3, SrZrO3, SrHfO3, BaTiO3, BaZrO3, BaHfO3, PbTiO3, PbZrO3 and PbHfO3 were calculated using a band theory based on the self-consistent-charge extended Huckel tight-binding (SCC-XHTB) method incorporating the relativistic effects except for the spin-orbit interaction in order to examine how the electronic structure changes due to the substitution of the A or B site atom. Moreover, calculations were also made for the tetragonal PbTiO3 to study the electronic structural changes due to the structural change from the cubic phase to the tetragonal one. It is shown that the upper valence band of all the AB O3's considered in the present paper consists of the 2p orbital of oxygen mixed with the d valence electrons of the B atom. The shape remains nearly the same in spite of the A or B site substitution. For the conduction band, it is shown that the bottom of which is made up from the nd t2g band of the B atom for SrTiO3, SrZrO3, SrHfO3, BaTiO3, BaZrO3, BaHfO3 and PbTiO3, but it originated from the O 2p orbital for PbZrO3 and PbHfO3. It is pointed out that the conduction band of the AB O3 is significantly modified by the change of the B atom, and that the modification is exceptionally remarkable in the case of the PbBO3. Accompanying the cubic-to-tetragonal structural change in PbTiO3, it is found that the charge transfer mainly occurs between the Pb 6p, Ti 3d and O 2p orbitals, and that the resultant charged states make the electronic structure which differs from that of the cubic phase. The differences are observed on the shape of the lower valence band, the width of the upper valence band and the energy gap.