  The phase diagram of cuprate high-temperature superconductors is investigated
on the basis of the three-band d-p model. We use the optimization variational
Monte Carlo method, where improved many-body wave functions have been proposed
to make the ground-state wave function more precise. We investigate the
stability of antiferromagnetic state by changing the band parameters such as
the hole number, level difference $\Delta_{dp}$ between $d$ and $p$ electrons
and transfer integrals. We show that the antiferromagnetic correlation weakens
when $\Delta_{dp}$ decreases and the pure $d$-wave superconducting phase may
exist in this region. We present phase diagrams including antiferromagnetic and
superconducting regions by varying the band parameters. The phase diagram
obtained by changing the doping rate $x$ contains antiferromagnetic,
superconducting and also phase-separated phases. We propose that
high-temperature superconductivity will occur near the antiferromagnetic
boundary in the space of band parameters.
