To provide a method of manufacturing a storage element efficiently and the storage element manufactured by that method.

In the manufacturing method of the storage element (for example, lithium ion secondary battery), a positive electrode 12, a negative electrode 14 and an electrolytic solution 30 are housed in a container 20 which is mainly constituted of a conductive material (SUS or the like), and the container 20 is sealed. Then, the external power supply 50 is connected between the container 20 and the negative electrode 14, and a voltage is applied in such conditions that an electric potential of the container 20 is approximately 1.23 V or more, and that the electric potential of the negative electrode 14 is approximately 0 V or less. By this, the container 20 becomes the positive electrode for electrolyzing, the negative electrode 14 becomes the negative electrode for electrolyzing, and water contained in this battery (assembly) 1 is electrically decomposed. Therefore water can be reduced (decomposed).