A redundancy circuit for a diode circuit composed of a plurality of diodes connected in series, according to the present invention, includes: bypass circuits, each of which is connected in parallel to each group, the diode circuit being divided into a plurality of group, each group having one or more diodes connected in series, and each bypass circuit including a first voltage-controlled switching device connected in parallel to the corresponding group, and a detection circuit which has a constant voltage device and a fusing circuit, both of which are connected in series, the detection circuit being connected in parallel to the corresponding group; and when, because of increased resistance of any diode, a voltage applied to one of the groups exceeds a given voltage defined by the constant voltage device, the voltage-controlled switching device is turned on so as to bypass a current that is to pass through the corresponding group, whereby continuing the operation of the whole apparatus even when one or more diodes are broken down.