  We investigate the phase structure of dense QCD using the Schwinger-Dyson
equation (SDE) with the improved ladder approximation in the Landau gauge. We
use the gluon propagator with the electric mode corrected by Debye screening
and the magnetic mode corrected by Landau damping. We solve the coupled SDE for
the Majorana masses of the quark and antiquark (separately from the SDE for the
Dirac mass) in the low density and intermediate density regions. In the low
density region, both the SDEs for the Majorana masses and the Dirac mass have
nontrivial solutions that correspond to the color symmetry breaking (CSB)
vacuum and the chiral symmetry breaking ($\chi$SB) vacuum. Comparing the values
of the effective potential in the two vacua, we show that the phase transition
from the $\chi$SB vacuum to the CSB vacuum is of first order. The resultant
value of the critical chemical potential is about 210 MeV, which is smaller
than that obtained from the SDE analysis including only the Dirac mass. The
momentum dependences of the Majorana masses of the quark and antiquark are
shown to be quite different, although the masses are of the same order.
