PURPOSE: To eliminate the influence of variance in the quantity of source light and variation of external light and to accurately detect a defective at any time by differentiating a video signal spatially and detecting the defective.

CONSTITUTION: A defective detecting circuit 2 inputs the video signal inputted by a television camera from an input terminal 5 and then sends out a detection signal from an output terminal 6 if detecting a defective. The video signal inputted from the terminal 5 is inputted to a buffer 7 and then inputted to a clamping circuit 8. A video signal outputted by the circuit 8 is inputted to delay elements 9aW9d in order. The elements 9aW9d consists of delay lines which delay the signals and the output signal of the circuit 8 is inputted to subtracting circuits 10aW10d at the same time. For example, the circuit 10a calculates the quantity of variation between the output video signal of the circuit 8 and the current video signal. The respective quantities of variation are inputted to comparators 11aW11d, which compares their inputs with threshold values set by variable resistors 12aW12d. If the quantities of variation exceeds the threshold values, signals are outputted to an OR circuit 14 through inverters 13aW13d and the signal is sent out from the terminal 6.