PURPOSE: To feed a DC power supply stably together with a battery in common use with respect to the distortion and drop of the waveform of a power supply voltage and the conspicuous decrease in power supply voltage by resetting a soft starting circuit and stopping a charging apparatus when the power supply voltage is abnormally decreased.

CONSTITUTION: A full-wave rectified voltage Es obtained from a three-phase full-wave rectifier circuit 23 is divided by resistors 24 and 25, and the voltages are inputted into a comparator 26 and compared with a reference voltage Er1. The output E0 of the comparator 26 is imparted into the A input of a multivibrator 27 having a time constant R2C2 28. When a voltage drop, whose time is longer than the time constant R2C2 is generated, the output Q of the multivibrator 27 is imparted to an input 19 for stopping the pulses of pulse generating parts 14, 15 and 16. The operation of a thyristor rectifier circuit 3 is stopped, and the charging apparatus is stopped. At the same time, a soft starting circuit 30, which slowly rises the reference voltage of a reference-voltage generator 29, is reset.