In a storage device state detection method in which the SOH or SOC of the storage device is inferred from the internal impedance of the storage device: the internal resistance of the storage device is measured by using a signal with a first frequency at which the internal impedance is reduced as a temperature is raised, and the initial SOH or initial SOC of the storage device is calculated from the measured value of the internal resistance; the internal impedance is measured by using a signal with a second frequency at which the internal impedance is increased as a temperature is raised, and the internal temperature of the storage device is calculated from the measured value of the internal impedance; and the SOH or SOC is inferred by using the calculated value of the internal temperature to correct the initial SOH or initial SOC.