  We study the phase structure of the unpolarized and polarized two-flavor
quark matters at zero and finite temperatures within the Nambu--Jona-Lasinio
(NJL) model. We focus on the region, which includes the coexisting phase of
quark-antiquark and diquark condensates. Generalizing the NJL model so as to
describe the polarized quark matter, we compute the thermodynamic potential as
a function of the quark chemical potential ($\mu$), the temperature ($T$), and
the polarization parameter. The result heavily depends on the ratio $G_D /
G_S$, where $G_S$ is the quark-antiquark coupling constant and $G_D$ is the
diquark coupling constant. We find that, for small $G_D / G_S$, the
"ferromagnetic" phase is energetically favored over the "paramagnetic" phase.
On the other hand, for large $G_D / G_S$, there appears the window in the
($\mu, T$)-plane, in which the "paramagnetic" phase is favored.
