A battery pack capacity adjusting device for adjusting a capacity of a battery pack having a plurality of secondary cells includes a control circuit board and a control section. The control circuit board is installable in the battery pack, and includes a capacity adjusting section to be electrically connected to the secondary cells to adjust a capacity of each of the secondary cells. The control section is configured to determine a number of the secondary cells whose capacities are adjustable together based on a relationship between a heat radiation amount of the control circuit board and a heat emission amount of the capacity adjusting section, and to control the capacity adjusting section to adjust the capacities of the number of the secondary cells that were determined to be adjustable together.