An electricity storage system comprising: a plurality of cartridges connected together in parallel, each cartridge comprising a plurality of cells connected together, and a circuit for preventing cross current, which restricts the current in each cartridge so as to cause the current to flow in one direction selected from a discharge direction and a charge direction, wherein said cartridges connected together in parallel are simultaneously charged with a direct current voltage converted from a commercial voltage, or said cartridges connected together in parallel are caused to discharge simultaneously so that said electricity storage system outputs a commercial voltage.