Abstract: Disclosed is a hydrated sodium-cobalt oxide exhibiting superconductivity at low temperatures. The hydrated sodium-cobalt oxide comprises a plurality of CoO2 layers each having edge-sharing CoO6 octahedra, and a combination of two water molecule layers and a single sodium ion layer, interposed between the adjacent CoO2 layers, wherein the distance between the adjacent CoO2 layers is in the range of 9.5 to 10.5 ?. The present invention provides the first cobalt oxide exhibiting superconductivity. The hydrated sodium-cobalt oxide has an extremely large distance between cobalt layers, and includes highly diffusible sodium ions and water molecules between the layers. Thus, based on high ability of ion-exchanging with various ion species and molecules, the hydrated sodium-cobalt oxide can be effectively used as a precursor for synthesizing materials through ion exchanging.