A deterioration diagnosis apparatus of a battery includes one or more processors. The one or more processors are configured to execute discharging of each of a plurality of cells included in an assembled battery while measuring a voltage of each of the cells, estimate a deterioration degree of each of the cells using voltage data indicating a transition of the voltage of each of the cells from a discharging start voltage to a predetermined discharging end voltage, and end the discharging when voltages of all the cells included in the assembled battery reach the predetermined discharging end voltage. The predetermined discharging end voltage is a cell voltage at which a change degree of the cell voltage per unit discharging amount starts to sharply rise while the cell voltage is dropping due to the discharging.