  Motivated by the colossal negative thermal expansion recently found in
BiNiO$_3$, the valence transition accompanied by the charge transfer between
the Bi and Ni sites is theoretically studied. We introduce an effective model
for Bi-$6s$ and Ni-$3d$ orbitals with taking into account the valence skipping
of Bi cations, and investigate the ground-state and finite-temperature phase
diagrams within the mean-field approximation. We find that the valence
transition is caused by commensurate locking of the electron filling in each
orbital associated with charge and magnetic orderings, and the critical
temperature and the nature of the transitions are strongly affected by the
relative energy between the Bi and Ni levels and the effective
electron-electron interaction in the Bi sites. The obtained phase diagram well
explains the temperature- and pressure-driven valence transitions in BiNiO$_3$
and the systematic variation of valence states for a series of Bi and Pb
perovskite oxides.
