Capacitor capacity diagnosis apparatus 1 includes a power source 3 for charging capacitor 2, a discharge circuit 4 connected in parallel to capacitor 2 for discharging the energy of capacitor 2, and a resistance for measuring a voltage drop during discharge. The circuit includes a voltage dividing circuit 5, a measuring circuit 6 that measures a divided voltage, and a diagnostic circuit 7 that determines whether the capacitor capacity is good or not based on a temporal change in voltage due to discharge. This makes it possible to diagnose the capacity of the capacitor of the power device in operation.
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