  Temperature vs. chemical-potential phase diagrams of an SO(5) model for
high-(T_c) cuprates are calculated by Monte Carlo simulation. There is a
bicritical point where the second-order antiferromagnetism (AF) and
superconductivity transition lines merge tangentially into a first-order line,
and the SO(5) symmetry is achieved. In an external magnetic field, the AF
ordering is first order in the region where the first-order melting line of
flux lattice joins in. There is a tricritical point on the AF transition line
from which the AF ordering becomes second order.
