A power source apparatus is provided with a plurality of battery modules connected in series, and a voltage detection circuit to detect the voltage of each battery module. Bypass-capacitors are connected at both ends of each voltage detection line to connect all adjacent voltage detection lines in series and form a series-connected line. A resistive element is connected between the bypass-capacitor at the battery module end of each voltage detection line and the battery module electrode terminal to connect one end of each voltage detection line to a battery module. A signal generator that outputs AC or pulse detection signals is connected to one end of the series-connected line while a signal detection circuit is provided at the other end of the series-connected line. Detection signals output from the signal generator are detected by the signal detection circuit to detect failure in the series-connected voltage detection lines.