Abstract: A second object is disposed so that the image of the second object is formed within a picture area obtained by picking up an image of a first object, formed by a microscope, by an area sensor but at a different position from a first portion of the image of the first object. An area, that includes a second portion of the second object having the same focusing point as that of the first portion of the first object of which image is to be processed, is set as the area for detecting a focusing point. A video signal of the image within the focusing-point detecting area is extracted from a video signal from the area sensor, and a focusing point of the area sensor on the second portion of the second object is detected based on a differentiating element of the extracted video signal. Then, the detected focusing point is obtained as a focusing point of the area sensor on the first portion of the first object of which image is to be processed.