The battery system has a battery block 2 with a plurality of battery cells 1 that are rectangular batteries stacked with intervening insulating separators 15, and that battery block 2 is held by fastening components 3. The fastening components 3 are provided with a pair of endplates 4 disposed at the ends of the stacked battery cells 1, and metal bands 5 on both sides of the battery block 2 connected at both ends to the endplates 4. An insulating separator 15 has an insulating plate section 15X, which intervenes between adjacent battery cells 1, and insulating walls 15C, which cover both battery cell 1 side-walls, formed from plastic as a single-piece. In this battery system, insulating plate sections 15X are sandwiched between battery cells 1, and the insulating walls 15C are disposed between battery cell outer side-walls and metal bands to insulate the battery cells from the metal bands.