A monitoring apparatus includes a detection circuit, a filter circuit, an amplifying circuit, a regulation circuit, a delay and charging circuit, and a driving circuit. The detection circuit receives a video signal, and performs an operation to obtain an image signal from the video signal. The filter circuit obtains an average intensity of a luminance signal corresponding to the image signal. The delay and charging circuit charges an input capacitor when receiving a low level regulated signal from the amplifying circuit. The driving circuit activates an alarm when a charging voltage of the chargeable capacitor exceeds a predetermined value.