  We investigate the ground-state phase diagram of the two-dimensional Hubbard
model based on the optimization variational Monte Carlo method. We use a wave
function that is an off-diagonal type given as $\psi=\exp(-\lambda
K)P_G\psi_0$, where $\psi_0$ is a one-particle state, $P_G$ is the Gutzwiller
operator, $K$ is the kinetic operator, and $\lambda$ is a variational
parameter. The many-body effect plays an important role as an origin of spin
correlation and superconductivity in correlated electron systems. We examine
the competition between the antiferromagnetic state and superconducting state
by varying the Coulomb repulsion $U$, the band parameter $t'$ and the electron
density $n_e$. We show a phase diagram that includes superconducting and
antiferromagnetic phases and that $t'=0$ is most favorable for
superconductivity.
