In this study, three types of Nasicon-type materials, Co3(PO4)2-CO2P2O7, Ni3(PO4)2-Ni2P2O7, and Cu3(PO4)2-Cu2P2O7, were synthesized as mixed-phase catalysts (MPCs) for evaluating their potential as new photocatalytic candidates (called Co3(PO4)2-CO2P2O7mpc, Ni3(PO4)2-Ni2P2O7mpc, and Cu3(PO4)2-Cu2P2O7mpc herein). Based on various physical properties, it was confirmed that there are two phases, M3(PO4)2 and M2P2O7, in which a similar phase equilibrium energy coexists. These colored powders showed UV and visible light responses suitable to our aim of developing 365-nm light-response photocatalysts for overall water-splitting. The photocatalytic performance of Ni2(PO4)3-Ni2P2O7 MPC showed negligible or no activity toward H2 evolution. However, Co2(PO4)3-Co2P2O7 MPC and Cu3(PO4)2-Cu2P2O7 MPC were determined as interesting materials because of their ability to absorb visible light within a suitable band. Moreover, an internal interface charge transfer was suggested to occur that would lower the recombination rate of electrons and holes. For Cu3(PO4)2-Cu2P2O7 MPC, the charge separation between the electron and hole was advantageously achieved, a water-splitting reaction was promoted, and hydrogen generation was considerably increased. The performance of a catalyst depended on the nature of the active metal added. In addition, the performance of the catalyst was improved when electrons migrated between the inter-phases despite the lack of a heterojunction with other crystals.