A battery assembly of a battery pack has a structure in which a substrate holder abuts on a side surface of a cell with a protecting circuit being held. The substrate holder includes a first side plate having a circuit part fitted part in which a circuit element of the protecting circuit is fitted and a second side plate along which a first lead is situated. Above the first side plate, a brim is provided that protrudes by an amount substantially equal to the sum of the thicknesses of the substrate and the circuit element, that is, the thickness of the protecting circuit substrate. Above the second side plate, a brim is provided that protrudes by an amount greater than the thickness of the first lead.