A capacitor capacitance diagnosis device includes a power supply which is for charging a capacitor, a discharge circuit which is connected to the capacitor in parallel to discharge energy of the capacitor, a resistance dividing circuit which is for measuring voltage drop value during discharging, a measurement circuit which measures divided voltage, and a diagnosis circuit which determines adequacy of capacitor capacitance from a time change in voltage due to the discharge. This makes it possible to diagnose adequacy of capacitor capacitance of an electric power apparatus during operation.