To detect unbalanced voltage in a DC generator by detecting the output voltage of a DC generator to be operated in series, computing a difference in the detected output power, deciding whether or not the computed difference is within a prescribed range, and outputting a prescribed signal if it is decided that the difference is not within the prescribed range.

This failure detector includes a voltage detector 2A detecting the output voltage V1 of a DC generator 1A, and a voltage detector 2B detecting the output voltage V2 of a DC generator 1B serial-connected to the DC generator 1A. A difference between the output voltage V1 detected by the voltage detector 2A and the output voltage V2 detected by the voltage detector 2B is computed by a subtracter 5A to decide whether or not the difference computed by the subtracter 5A is within the prescribed range. If it is decided that the difference is not within the prescribed range, a failure signal AS annunciating failure is outputted by an upper/lower limit comparison circuit 7A. It is thus possible to detect unbalanced voltage in the DC generator operated in series.