Even if requirements for any scale of system constructions occur, a battery system of flexibly responding to the requirements is provided. The battery system comprising a battery module having a plurality of battery cells being connected, a battery pack having a plurality of battery modules being connected either in series or in parallel or both in series and in parallel, and a battery block having a plurality of battery packs being connected either in series or in parallel or both in series and in parallel. They are mutually layered. In the battery system, the battery module, the battery pack, and the battery block are previously prepared as variations of layered basic units. In response to requirements for any scale of system constructions, the basic units are combined accordingly.