Capacitor device life diagnosis method

In an initial charging time when a predetermined bus line voltage is formed on a DC bus line by a converter circuit when a motor control device to which an electricity storage device is connected is powered on, initial charging of a capacitor device is performed by allowing a step-down switching element of a step up/down chopper circuit in the electricity storage device to perform an on/off operation at a predetermined number of times. The actual capacitance of the capacitor device is calculated from: the total energy given to the capacitor device within the period of the on/off operation performed at the predetermined number of times; and the charging voltage that the capacitor device indicates. The calculated actual capacitance and the initial capacitance of the capacitor device are compared with each other, thereby estimating the degree of degradation of the capacitor device and diagnosing the life thereof.