We investigate the ground Tank Tops state of strongly correlated electron systems based on an optimization variational Monte Carlo method to clarify the mechanism of high-temperature superconductivity.The wave function is optimized by introducing variational parameters in an exponential-type wave function beyond the Gutzwiller function.The many-body effect plays an important role as an origin EMPTY VEG CAPS "00" of superconductivity in a correlated electron system.There is a crossover between weakly correlated region and strongly correlated region, where two regions are characterized by the strength of the on-site Coulomb interaction U.We insist that high-temperature superconductivity occurs in the strongly correlated region.