Layered triangular organic dimer Mott (DM) insulator κ -(ET)2 Cu2 (CN)3 was shown to exhibit a relaxor-like dielectric anomaly below 40 K with strong dispersion relation, reflecting its electric dipole glass (ferroelectric charge order; FCO) nature[1, 2]. The dielectric anomaly in κ -(ET)2 Cu2 (CN)3 indicates that this compound is located in the vicinity of the DM-FCO phase boundary, where ferroelectric fluctuation such as the electric dipole glass state or the polar cluster is formed in the DM phase. Optical excitation of the DM-FCO competing state by an ultrashort light pulse enables us to achieve dramatic responses, such as photoinduced ferroelectricity, photoinduced growth of the electric dipole glass or the polar clusters.