A controller of a deterioration estimation device for a secondary battery is configured to acquire a first open circuit voltage when a temperature of the secondary battery is a first temperature and a second open circuit voltage when the temperature of the secondary battery is a second temperature changed from the first temperature, and estimate a degree of deterioration of the secondary battery by using a difference between first and second slopes. The first slope is a ratio of a voltage change obtained by subtracting the first open circuit voltage from the second open circuit voltage to a temperature change obtained by subtracting the first temperature from the second temperature. The second slope is a ratio of the voltage change to the temperature change when the temperature of the new secondary battery has changed from the first temperature to the second temperature.