To provide a detector for automatically detecting the welding of contacts of an electromagnetic contactor formed in a drive circuit for a driving motor of a hoist crane or the like or other loads, and failure of a push-button switch for switching the electromagnetic contactor.

This failure detector comprises: a load current detection means 1 for detecting a current flowing through a load to generate a load current signal; an excitation current detection means 2 for detecting a current flowing through a coil of the electromagnetic contactor to generate an excitation current signal; a power-on detection means 3 for detecting the switching of a main electromagnetic contactor MC0 by a power switch PB1 to generate a power-on signal; a welding detection means 4 of the electromagnetic contactor for comparing the load current signal generated by the means 1 with the excitation current signal generated by the means 2; and a failure detection means 5 of the push-button switch for comparing the load current signal generated by the means 1 with the power-on signal generated by the means 3. In the failure detector, the means 5 has a function for detecting the failure by pressing and operating the switch PB1 of a pendant switch and the push-button switch PB for operating a failure detection object at the same time to turn off the excitation current signal generated by the means 2.