A fault sensor device comprising a receiver (3; 3A; 3B) for receiving microwaves (2, 8) radiated from an object to be monitored (1; 1A; 1B; 1C; 1D; 1E; 1F), a detector (4) connected to the receiver (3; 3A; 3B) for converting and outputting the microwaves (2, 8) into a video signal (2S4; 2S21), and a signal processor (5) for converting the video signal (2S4, 2S21) output by the detector (4) into a presentation signal (2S5) and outputting the presentation signal (2S5) to an external presentation device (91; 91A), and for determining whether or not the microwaves (2, 8) received are associated with a fault, based on one of the duration and the frequency of the video signal (2S4; 2S21) and the level of the video signal, and for issuing an alarm signal (2S6) when the determination reveals that the microwaves are associated with the fault.