A battery module 100 includes a plurality of cells aligned and accommodated in a case 10, wherein a positive electrode external terminal 20 and a negative electrode external terminal 21 which are connected to electrodes of the plurality of cells are disposed in parallel on a first side surface 11 of the case 10 at a predetermined interval, a pair of recessed sections 30, 31 are formed on a second side surface 12 adjacent to the first side surface 11 of the case 10 at a same interval as the predetermined interval, a first portion 51 of an L-shaped electrode piece 50 is selectively attachable to the positive electrode external terminal 20 or the negative electrode external terminal 21, and a second portion 52 of the electrode piece 50 is selectively attachable to any one of the pair of recessed sections 30, 31.