A battery pack comprising: a plurality of stacked battery modules, wherein each battery module includes: a plurality of cells aligned and accommodated in a case, wherein a positive electrode external terminal and a negative electrode external terminal which are connected to electrodes of the plurality of cells are disposed in parallel on a first side surface of the case at a predetermined interval, a pair of recessed sections (30, 31) are formed on a second side surface adjacent to the first side surface of the case at a same interval as the predetermined interval, and a first portion of an L-shaped electrode piece is selectively attachable to the positive electrode external terminal or the negative electrode external terminal, and a second portion of the electrode piece is selectively attachable to the corresponding recessed section (30, 31), characterized in that the battery modules are stacked in a direction perpendicular to the second side surface, the first portion of the electrode piece is attached to one of the positive electrode external terminal or the negative electrode external terminal which is disposed on the first side surface of the case of the battery module at an uppermost level, the second portion of the electrode piece is attached to one of the pair of recessed sections (30, 31) formed on the second side surface of the case of the battery module at the uppermost level, and the second portion of the electrode piece serves as an external terminal of one of electrodes of the battery pack. a plurality of stacked battery modules, wherein each battery module includes: a plurality of cells aligned and accommodated in a case, wherein a positive electrode external terminal and a negative electrode external terminal which are connected to electrodes of the plurality of cells are disposed in parallel on a first side surface of the case at a predetermined interval, a pair of recessed sections (30, 31) are formed on a second side surface adjacent to the first side surface of the case at a same interval as the predetermined interval, and a first portion of an L-shaped electrode piece is selectively attachable to the positive electrode external terminal or the negative electrode external terminal, and a second portion of the electrode piece is selectively attachable to the corresponding recessed section (30, 31), characterized in that the battery modules are stacked in a direction perpendicular to the second side surface, the first portion of the electrode piece is attached to one of the positive electrode external terminal or the negative electrode external terminal which is disposed on the first side surface of the case of the battery module at an uppermost level, the second portion of the electrode piece is attached to one of the pair of recessed sections (30, 31) formed on the second side surface of the case of the battery module at the uppermost level, and the second portion of the electrode piece serves as an external terminal of one of electrodes of the battery pack. a plurality of cells aligned and accommodated in a case, wherein a positive electrode external terminal and a negative electrode external terminal which are connected to electrodes of the plurality of cells are disposed in parallel on a first side surface of the case at a predetermined interval, a pair of recessed sections (30, 31) are formed on a second side surface adjacent to the first side surface of the case at a same interval as the predetermined interval, and a first portion of an L-shaped electrode piece is selectively attachable to the positive electrode external terminal or the negative electrode external terminal, and a second portion of the electrode piece is selectively attachable to the corresponding recessed section (30, 31), a positive electrode external terminal and a negative electrode external terminal which are connected to electrodes of the plurality of cells are disposed in parallel on a first side surface of the case at a predetermined interval, a pair of recessed sections (30, 31) are formed on a second side surface adjacent to the first side surface of the case at a same interval as the predetermined interval, and a first portion of an L-shaped electrode piece is selectively attachable to the positive electrode external terminal or the negative electrode external terminal, and a second portion of the electrode piece is selectively attachable to the corresponding recessed section (30, 31), characterized in that the battery modules are stacked in a direction perpendicular to the second side surface, the first portion of the electrode piece is attached to one of the positive electrode external terminal or the negative electrode external terminal which is disposed on the first side surface of the case of the battery module at an uppermost level, the second portion of the electrode piece is attached to one of the pair of recessed sections (30, 31) formed on the second side surface of the case of the battery module at the uppermost level, and the second portion of the electrode piece serves as an external terminal of one of electrodes of the battery pack.