The development of solid electrolytes with Mg-ion conductivity at room temperature is an important issue to achieve all-solid magnesium batteries. We focus on organic ionic crystals with Mg-ion conduction paths in addition to nonflammable and nonvolatile features as an innovative candidate of solid electrolytes with Mg-ion conductivity. Herein, we show the development of novel organic ionic crystals, [N(CH3)4-n(CH2CH3)n][Mg{N(SO2CF3)2}3] (n = 0 or 2), using analogs of ionic liquids, [N(CH3)4][N(SO2CF3)2] (N1111TFSA) and [N(CH3)2(CH2CH3)2][N(SO2CF3)2] (N1122TFSA), and magnesium salt, Mg{N(SO2CF3)2}2 (Mg(TFSA)2). We also report the crystal structures of the obtained crystals and the high Mg-ion conductivity of 10-4 S cm-1 under mild conditions of 80 °C in the solid state. These results indicate that organic ionic crystals with ion conduction paths have significant potential as safe solid electrolytes and provide insights into developing innovative Mg-ion conductors.