To provide the fault detector in the supervisory device where a supervisory load on supervising personnel is relieved without the need for an exclusive fault sensor.

The fault detector (image data receiver) 11 of this supervisory device supervises a supervised object based on a supervisory image supplied from an object photographing section that photographs the prescribed supervised object through a network and displays the supplied supervisory image on a display device 13 and allows an output section 15 to raise a warning tone in response to a change in a supervisory image when the supervised object is subjected to change. The fault detector 11 receives moving image data compressed by a prescribed image data compression technology such as the JPEG, and a notice signal generating section 29 generates a notice signal that changes with a change in a data size of moving image data by one frame.