A deterioration degree determination device includes a charging and discharging control unit, a battery characteristic acquisition unit, and a determination unit. The charging and discharging control unit, in a state where multiple secondary batteries are connected to each other to form a battery pack, performs a charging and discharging operation of the battery pack while voltages of the secondary batteries are individually measured. The battery characteristic acquisition unit acquires a battery characteristic related to transition in a battery state over a predetermined voltage section for at least some of the multiple secondary batteries. The determination unit determines a deterioration degree of at least some of the multiple secondary batteries based on the battery characteristic or a battery characteristic relationship value calculated based on the battery characteristic.