[en] Motivated by recent experiments in κ-(BEDT-TTF)₂Cu₂(CN)₃, we studied dielectric and magnetic properties of a dimer-Mott insulator. We derived the effective Hamiltonian where the number of electrons per dimer is taken to be one. Charge and magnetic structures at a finite temperature were obtained by using the mean-field and Monte-Carlo methods. Magnetic and ferroelectric phases are exclusive with each other. Dielectric fluctuation is remarkable near the phase boundary between the dimer-Mott and ferroelectric phases. Implications of the present results for κ-(BEDT-TTF)₂Cu₂(CN)₃ were discussed. (author)