A method of estimating a deteriorated state of a battery includes steps S102 to S110. S102 is a step of obtaining a voltage and a current of the battery a plurality of times for a data acquisition period. S104 is a step of calculating an amount of change in current, an amount of change in temperature, and an amount of change in SOC during the data acquisition period. S106 is a step of obtaining an allowable amount of change in current, an allowable amount of change in temperature, and an allowable amount of change in SOC based on an average temperature. S110 is a step of calculating an impedance component for each frequency bandwidth based on the voltage and the current by subjecting the voltage and the current to Fourier transform when all amounts of change are smaller than the allowable amounts of change.