  Phase structure and phase transitions in dense QCD are studied using the
Schwinger-Dyson (SD) method in the improved ladder approximation. We construct
the Cornwall-Jackiw-Tomboulis (CJT) effective potentials at finite temperature
for two types of pairing ansatz, namely the Color-Flavor locking (CFL) state
and the two flavor superconducting (2SC) state. Strong coupling effects at low
densities, such as the off-Fermi surface and antiquarks contribution to the
pairing gap due to the large effective coupling, make the gap, critical
temperature and those ratio deviate from those weak coupling values.
Nevertheless, the ratio of the physical quantity for the CFL and that for the
2SC does not differ so much from the weak coupling value, as long as the
pairing interaction is taken to be the same for the both cases. As a
consequence, the CFL state always dominates over 2SC state and the critical
temperatures to quark-gluon plasma (QGP) from both states coincides in the
chiral limit. The energy gain in the CFL state relative to the 2SC state gets
smaller towards the critical line dividing (mu, T) plane into the QGP and CFL
phases, and thus small perturbations can remove the degeneracy of these
critical lines. As one of such perturbations, the effect of the strange quark
mass m_s on the quark-pairing is examined. In particular, using a simple
kinematical criterion, we discuss the behaviour of the ``color-flavor unlocking
line'' on which the CFL phase turns into the 2SC phase, against the variation
of m_s.
