PURPOSE: To automatically adjust the center position of a limiting value by storing the position of a detecting object in a specific numbers in order only when the position of the detecting object is within the limiting value an renewing the mean value as the center position.

CONSTITUTION: The allowable value of the specific width of plus and minus with the center position as the center and of the center position of a limiting value is input to a visual sensor 10, the limiting value is set first, and when the devices of a robot, etc., connected to the visual sensor and the visual sensor 10 are operated, the visual sensor 10 picks up a detecting object by a camera 22, finding the position of the object, deciding as to whether or not the position of the object is within the limiting value and in case of within the limiting value, a correction data is transmitted to the device of the robot, etc. Also the position of the detected object is stored in a memory 20. Then, whenever the position of the object stored in the memory 20 reaches a specific number or the position of the object is stored in the memory 20 in order, the mean value is calculated and renewed as the center position of the limiting value.