Remaining capacity calculation section (110) calculates remaining capacities for each of cells (300-1) and (300-2) connected in parallel with each other, and control section (130) discharges one of cells (300-1) and (300-2) having priority until the remaining capacity of that cell becomes equal to a second threshold value stored in storage section (120), if the remaining capacity of the one of the cells calculated by remaining capacity calculation section (110) is equal to a first threshold value stored in storage section (120).