  Two-dimensional t-J model is studied by a variational Monte Carlo method,
using Gutzwiller-Jastrow-type wave functions. Various kinds of superconducting
pairing symmetries are compared in order to determine the phase diagram of the
ground state in the full J/t-n plane. Near the half filling where the high
temperature superconductivity is expected, the d_{x^2-y^2} wave pairing state
is always the most stable among various symmetries. The three-site term hardly
changes the phase diagram in this regime. In the low electron density, the
extended s-type wave becomes a quantitatively good state for large J/t,
although the energy gain is small. The Gutzwiller wave function is shown to be
the exact ground state in the low-electron-density limit for the supersymmetric
case (J/t=2).
