a threshold voltage level during a period of there on states simultaneously it can be judged with the clamp diode dn 1 having changed into the short circuit state fig2 shows the current route of a short circuit current when the clamp diode dn 2 in a switching leg changes into a short circuit state as shown in fig2 when the switching element q 6 turns on a short circuit current flows by the route of u2212 e 3 potential terminal to dn 2 q 6 and u2212 e potential terminal therefore when the voltage vce of the switching element q 6 exceeds a threshold voltage level during a period of its on state it can be judged with the clamp diode dn 2 having changed into the short circuit state as mentioned above it becomes possible like the case of three level inverter in the second embodiment what is necessary to detect the over current failure of a clamp diode is just to detect that any of the voltage vce of both ends of switching element exceeding the threshold voltage level during a period of there on states if the voltages vce of adjoined switching elements of both ends are also supervised during a period of there on states the part which changed into a short circuit state may be specified in the case of n level inverter the clamp diode which has made a short circuit failure may be specified by supervising voltages vce of n 3 adjoined switching elements of both ends fig2 shows one example of the internal circuit diagram of a logic operation circuit 7 c used for the power converter concerning the fourth embodiment of this invention to the same parts of the fourth embodiment.