PURPOSE: To stabilize a focusing operation by making the detection region of a sensor large on a telephoto side and small on a wide angle side.

CONSTITUTION: When a zoom lens is driven to zoom, the distance information up to an object is detected by a focusing length detection circuit 4, and the distance information is inputted to a sensor area changing circuit 6. The circuit 6 outputs the sensor area information corresponding to each focal length to an automatic focusing sensor 7 in accordance with the dinstance information inputted thereto. An image to be picked up past a photographing lens 1 is formed an image on an image pikup plane 10a, and a part thereof is formed an image on the sensor 7 as well by a beam splitter 9. Since the sensor 7 is limited of the detection region in accordance with the sensor area information from the circuit 6, the sensor outputs a part of the image to be picked up formed the image by the splitter 9 to a processing circuit 11 which emits a focusing signal to a motor driving circuit 12. A motor 13 is run by said circuit to drive and rotate a focusing ring so that the automatic focusing operation for the image to be picked up in the limited detection region is accomplished.