A method of estimating a deteriorated state of a secondary battery performed by a controller mounted on a vehicle, the method comprising: obtaining a plurality of times, a voltage, a current, and a temperature of the secondary battery during a prescribed period and storing the voltage, the current, and the temperature in a memory; calculating an amount of change in current of the secondary battery, an amount of change in temperature of the secondary battery, and an amount of change in SOC of the secondary battery during the prescribed period; obtaining from the memory, an allowable amount of change in current, an allowable amount of change in temperature, and an allowable amount of change in SOC, which are determined in advance for each temperature, each current, or each SOC of the secondary battery during the prescribed period and stored in the memory, based on the temperature, the current, or the SOC of the secondary battery, the allowable amount of change in current representing an allowable upper limit of the amount of change in current, the allowable amount of change in temperature representing an allowable upper limit of the amount of change in temperature, the allowable amount of change in SOC representing an allowable upper limit of the amount of change in SOC; carrying out, by Fourier transform, frequency conversion of the voltage and the current during the prescribed period of the secondary battery stored in the memory and calculating an impedance for each frequency band of the secondary battery based on the frequency-converted voltage and current when all of a current condition, a temperature condition, and an SOC condition are satisfied, the current condition being a condition that the amount of change in current is smaller than the allowable amount of change in current, the temperature condition being a condition that the amount of change in temperature is smaller than the allowable amount of change in temperature, the SOC condition being a condition that the amount of change in SOC is smaller than the allowable amount of change in SOC; and estimating whether the secondary battery is in a high-rate deterioration state by using a ratio between an impedance corresponding to a prescribed frequency band and an impedance corresponding to another frequency band higher than the prescribed frequency band and a reference value, the prescribed frequency band being a frequency band in which at least a direct-current resistance and a reaction resistance mainly contribute, wherein high-rate deterioration refers to deterioration caused by an uneven concentration of salt in an electrode assembly as a result of continued charging and discharging at a high current. obtaining a plurality of times, a voltage, a current, and a temperature of the secondary battery during a prescribed period and storing the voltage, the current, and the temperature in a memory; calculating an amount of change in current of the secondary battery, an amount of change in temperature of the secondary battery, and an amount of change in SOC of the secondary battery during the prescribed period; obtaining from the memory, an allowable amount of change in current, an allowable amount of change in temperature, and an allowable amount of change in SOC, which are determined in advance for each temperature, each current, or each SOC of the secondary battery during the prescribed period and stored in the memory, based on the temperature, the current, or the SOC of the secondary battery, the allowable amount of change in current representing an allowable upper limit of the amount of change in current, the allowable amount of change in temperature representing an allowable upper limit of the amount of change in temperature, the allowable amount of change in SOC representing an allowable upper limit of the amount of change in SOC; carrying out, by Fourier transform, frequency conversion of the voltage and the current during the prescribed period of the secondary battery stored in the memory and calculating an impedance for each frequency band of the secondary battery based on the frequency-converted voltage and current when all of a current condition, a temperature condition, and an SOC condition are satisfied, the current condition being a condition that the amount of change in current is smaller than the allowable amount of change in current, the temperature condition being a condition that the amount of change in temperature is smaller than the allowable amount of change in temperature, the SOC condition being a condition that the amount of change in SOC is smaller than the allowable amount of change in SOC; and estimating whether the secondary battery is in a high-rate deterioration state by using a ratio between an impedance corresponding to a prescribed frequency band and an impedance corresponding to another frequency band higher than the prescribed frequency band and a reference value, the prescribed frequency band being a frequency band in which at least a direct-current resistance and a reaction resistance mainly contribute, wherein high-rate deterioration refers to deterioration caused by an uneven concentration of salt in an electrode assembly as a result of continued charging and discharging at a high current.