[Object] To provide to a storage device state detection method of accurately detecting the state of health (SOH) and state of charge (SOC) of a storage device. [Solution] In a state detection method, for a storage device (1), in which the SOH or SOC of the storage device (1) is inferred from the internal impedance of the storage device (1): the internal resistance of the storage device (1) is measured by using a signal with a first frequency at which the internal impedance of the storage device (1) is reduced as a temperature is raised, and the initial SOH or initial SOC of the storage device (1) is calculated from the measured value of this internal resistance; the internal impedance of the storage device (1) is measured by using a signal with a second frequency at which the internal impedance of the storage device (1) is increased as a temperature is raised, and the internal temperature of the storage device (1) is calculated from the measured impedance value of this 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.