PURPOSE: To immediately detect a runaway of a load caused by a failure of a switching means or the like to prevent occurrence of a trouble by a method wherein a terminal, voltage signal of the switching means is inputted to a failure detection means to be compared with a detection level reference signal that is changed by a detection level changing means correspondingly to a power supply control.

CONSTITUTION: A failure detector comprises a triac 108 conducting supply/ interruption of an electric power from a power source 104 to a load 105; a control part 101 controlling an electric energy to be supplied to the load 105; a diode bridge 110 detecting the terminal voltage of the triac 108; and a comparator 113 comparing a DC-level signal VTR obtained by converting an output signal from the diode bridge 110 with a detection level reference signal. Vref. Furthermore, it comprises an AND gate 115 receiving an output signal from the comparator 113 and a phase control timing signal and issuing a triac error signal ERR to the control part 101 and a reference signal generation circuit 121 changing a detection level reference signal Vref correspondingly to the power supply control of the control part 101 to the load 105.