  It is important to understand the mechanism of high-temperature
superconductivity. It is obvious that the interaction with large energy scale
is responsible for high critical temperature $T_c$. The Coulomb interaction is
one of candidates that bring about high-temperature superconductivity because
its characteristic energy is of the order of eV. There have been many works for
the Hubbard model including three-band d-p model with the on-site Coulomb
repulsion to investigate a possibility of high-temperature superconductivity.
It is, of course, not trivial whether the on-site Coulomb interaction leads to
a pairing interaction between two electrons. We argue that high-temperature
superconductivity is possible in the strongly correlated region by using the
variational Monte Carlo method for the two-dimensional t-U-J-V model. The
exchange interaction J and the nearest-neighbour attractive interaction V
cooperate with U and will act to enhance the critical temperature.
