A monitoring system is applied to an electrical system, which includes a DC power source and an electrical apparatus that is connected to the DC power source by a pair of electric power leads incorporating respective switches. To detect leakage current from the DC power source, a low-frequency AC signal is applied via a large-capacitance capacitor to a specific connection position, between a first terminal of the DC power source and the corresponding switch, and the resultant voltage of that signal is measured. To detect a short-circuit failure of one or both of the switches, a high-frequency AC signal is applied via a low-capacitance capacitor to the specific connection position, and the resultant signal voltage is measured. Judgement as to occurrence of leakage current and/or short-circuit failure is based on the measured signal voltage values.