A charged voltage between the ground and a positive pole of backup capacitor 5 and a ground-casing voltage developed between the ground and a casing of capacitor 5 are determined. When the ground-casing voltage is smaller than a first value, it is judged that capacitor 5 has an opened circuit on its positive pole side; when the ground-casing voltage is larger than a second value, it is judged that capacitor 5 has an opened circuit on its negative pole side. A difference between the ground-casing voltage and the charged voltage is divided by the ground casing voltage so that a coefficient is obtained. When the coefficient is outside a predetermined range of values, it is judged that capacitor 5 is poor in capacitance, which makes it possible to determine, even in operation, whether capacitor 5 functions properly.