The present invention provides a modular system for accommodating batteries therein, which has a simple structure and a high reliability and prevents a difference in properties between the batteries included therein. The modular system includes a plurality of single batteries 1, which are arranged side by side between the beams of a top plate 21 and a bottom plate 22 to locate covers of the batteries 1 with terminals on a front side of the top plate 21 and the bottom plate 22. The single battery 1 has a plurality of ribs 12 that are formed on a specific pair of opposite outer surfaces in such a manner as to extend in a longitudinal direction of the single battery 1, and have concaves formed at regular intervals. The plurality of ribs 12 formed on each outer surface of each single battery 1 are in contact and joined with the plurality of ribs 12 formed on a matching outer surface of an adjoining single battery 1. The joint surfaces of the adjacent single batteries 1 form connection grooves 13 that are defined by the concaves of the ribs 12 and extend perpendicularly to the top plate 21 and the bottom plate 22.