To detect failures in a plurality of rectifying smoothing portions with a failure detector for a dc-dc converter which outputs high voltage, and to prevent spread of hindrance.

This is a failure detector for a dc-dc converter which on/off- controls currents flowing in primary windings n11, n12 of transformers T1, T2 by switching transistors Q1, Q2, has rectifying smoothing portions RE1, RE4 which rectify and smooth voltages induced in secondary windings n21-n24 of the transformers T1, T2, and outputs a high voltage obtained by connecting a plurality of these rectifying smoothing portions RE1-RE4 in series and adding output voltages V1-V4 of individual smoothing portions RE1-RE4. The detector is provided with a voltage detecting means for detecting the respective output voltages V1-V4 of the smoothing portions RE1-RE4 using zener diodes ZD1-ZD4, etc., and a failure detecting comparator CMP1, which compares a detection signal representing malfunctioning time of the output voltages V1-V4 by these voltage detector means, with a reference voltage Vr1, which corresponds to value at normal time and outputs an alarm signal.