  We construct a low energy effective Lagrangian for the two-color QCD
including the "vector" bosons (mesons with J^P=1^- and diquark baryons with
J^P=1^+) in addition to the pseudo Nambu-Goldstone bosons with a degenerate
mass M_\pi (mesons with J^P=0^- and baryons with J^P=0^+) based on the chiral
symmetry breaking pattern of SU(2N_f) \to Sp(2N_f) in the framework of the
hidden local symmetry. We investigate the dependence of the "vector" boson
masses on the baryon number density \mu_B. We show that the \mu_B-dependence
signals the phase transition of U(1)_B breaking. We find that it gives
information about mixing among "vector" bosons: e.g. the mass difference
between \rho and \omega mesons is proportional to the mixing strength between
the diquark baryon with J^P=1^+ and the anti-baryon. We discuss the comparison
with lattice data for two-color QCD at finite density.
