PURPOSE: To always operate a monitoring device within an appropriate operation range by adjusting the number of fetching fields and the gain when a level of brightness data of a prescribed monitoring area becomes too large or too small.

CONSTITUTION: When the number of fetching fields FR is subjected to '-1' decrement processing, a value of monitoring brightness data becomes low by decreasing the number of times of cumulative addition of the monitoring brightness data to be in a state that the monitoring brightness data is restored from an excessive state to an appropriate operation range, and when a negative result is obtained, the value is in a state that the number of fetching fields F is the lowest limit value F=1, and such control as the number of fetching fields F is furthermore lowered cannot be executed. In this case, a CPU 36 lowers the gain by a prescribed quantity by applying a gain adjusting signal S2 to a gain adjusting circuit 13. As a result, by lowering a signal level of a video input signal VDIN obtained in an output terminal of a camera switching circuit 12, the value of monitoring brightness data can be restored to an appropriate operation range from an excessive state in accordance with a level of brightness data DB fetched to an image information memory 27.