This invention is a diode defect detecting device including a current detector for detecting the primary current of a transformer, a comparator for comparing the detection current detected by the current detector with a current reference and outputting a reset signal if the detection current is larger than the current reference, an oscillator for generating a clock signal, a flip-flop circuit for receiving a set signal on the basis of a front and an end edge of the clock signal generated by the oscillator and receiving the output reset signal from the comparator, a polarity changing circuit for outputting a polarity changing signal for changing the polarities of the plurality of diodes on the basis of the output clock signal from the flip-flop circuit, and a determination circuit for calculating, in order to detect malfunctions of the plurality of diodes, any impedance change on the secondary side viewed from the primary side of the transformer on the basis of the pulse width of each polarity of the output clock signal from the flip-flop circuit.