  We investigate hadron spectra in 2-color QCD using lattice simulation with
$N_{f}=2$ at low temperature and finite density in which there appears not only
the hadronic phase but also the superfluid phase. We first calculate the pion
and rho meson spectrum, which is well-known from previous works. The spectral
ordering of these mesons flips around the quark chemical potential
$\mu=m^{0}_{\pi}/2$ ($m^{0}_{\pi}$: the pion mass at $\mu=0$), where the phase
transition between the hadronic and superfluid phases occurs. For $\mu \gtrsim
m^{0}_{\pi}/2$, the effective mass for the pion linearly increases while the
one for the rho meson monotonically decreases. Furthermore, we measure hadron
spectra with the isospin $I=0$ and the angular momentum $J^{P}=0^{\pm}$. The
effective masses for the meson, diquark, and antidiquark with the same quantum
number become degenerate just below $\mu = m^{0}_{\pi}/2$, and the three
hadrons have the same mass in the superfluid phase. It suggests that mixing
occurs between spectra associating with mesons and baryons due to the
$U(1)_{B}$ symmetry breaking. This phenomenon can be explained in the linear
sigma model with the approximate $SU(4)$ Pauli-Gursey symmetry.
