[en] 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_π (mesons with J^P=0^- and baryons with J^P=0⁺) based on the chiral symmetry breaking pattern of SU(2N_f)→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 μ_B. We show that the μ_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 'ρ' and 'ω' mesons is proportional to the mixing strength between the diquark baryon with J^P=1⁺ and the antibaryon. We discuss the comparison with lattice data for two-color QCD at finite density.