  We investigate the phase structure of hot and/or dense QCD with massless
2-flavors using the Schwinger-Dyson equation (SDE) with the improved ladder
approximation in the Landau gauge. We examine the effect of the antiquark
contribution and find that setting the antiquark Majorana mass equal to the
quark one is a good approximation in the medium density region. The imaginary
part of the Dirac mass has an important influence on the chiral phase
transition, in particular on the tricritical point.
