PURPOSE: To improve the precision of measurement of a principal optical axis by controlling the quantity of light taken into a television camera so that the area of a closed curved surface having an equal quantity of illumination coincides with a target area or controlling the quantity of input of a video signal from the television camera.

CONSTITUTION: The video signal of a television camera (anyone of 1, 2a, or 2b) designated by a camera selecting signal is inputted to an area operating circuit 34 to calculate the area of a hot zone. The geometrical center of the hot zone is obtained by a geometrical center operating circuit 36 to output a principal optical axis position signal 50. Meanwhile, the calculated output of the area operating circuit 34 and a set value which is set by a setter 38 and indicates a target area are compared with each other by a comparator 40, and a comparison signal corresponding to the difference is outputted as a driving signal to a lens disphragm driving mechanism part (one of mechanism parts 18aW18c). As the result, the lens diaphragm driving part of the pertinent television camera is driven until the comparison signal of the comparator 40 becomes zero, thus performing correction automatically so that the area of the hot zone coincides with the target area.