We investigate the phase structure of hot and/or dense QCD using the Schwinger-Dyson approach with the improved ladder approximation in the Landau gauge. The QCD phase diagram is drawn including the chiral symmetry breaking phase and the color-superconducting phase. In this approach, there are two remarkable features: the critical temperature of the color-superconductivity is about 100 MeV, and the triciritical point on the chiral phase transition line has the value of the chemical potential with about 20 MeV. We examine the effect of the antiquark contribution on the color supercoductivity and find that setting the antiquark Majorana mass equal to the quark one is a good approximation in the medium density region. We also discuss the effect of the imaginary part of the Dirac mass on the chiral phase transition at finite temperature $T$ and finite chemical potential $\mu$.