  It is very important to elucidate the mechanism of superconductivity for
achieving room temperature superconductivity. This paper is a short review
article on the mechanism of high-temperature superconductivity. In the first
half of this paper, we give a brief review on mechanisms of superconductivity
in many-electron systems. We believe that high-temperature superconductivity
may occur in a system with interaction of large-energy scale. Empirically, this
is true for superconductors that have been found so far. In the second half of
this paper, we discuss cuprate high-temperature superconductors. We argue that
superconductivity of high temperature cuprates is induced by the strong on-site
Coulomb interaction, that is, the origin of high-temperature superconductivity
is the strong electron correlation. We show the results on the ground state of
electronic models for high temperature cuprates on the basis of the
optimization variational Monte Carlo method. A high-temperature superconducting
phase will exist in the strongly correlated region.
