An inorganic electride, 12CaO center dot 7Al(2)O(3):e(-), synthesized by exclusively replacing oxygen ions in the sub-nanometer-sized cages of 12CaO center dot 7Al(2)O(3) crystal with electrons exhibits a superconducting transition at a temperature (T-c) of similar to 0.4 K. T-c varies in the range of 0.14-0.4 K with the concentration of anionic electrons, which are primarily distributed over crystallographic spaces without occupying any particular framework ions. The precursor of electride is composed of representative metal oxides, which are electrical insulators. Thus, the exotic crystal structure of electrides provides an insight for a material platform to realize a new superconductor.