1. An image capturing device comprising: an image capturing unit configured to capture an image; a detection unit configured to detect a shape of an image area of an observed subject to be focused in the image captured by the image capturing unit; an area setting unit configured to set one or more contrast evaluation areas according to the shape of the image area detected by the detection unit; a focusing unit configured to cause the image capturing unit to focus based on image data in the contrast evaluation areas set by the area setting unit; and a determining unit configured to determine numbers of the contrast evaluation areas in vertical and horizontal directions according to length ratios in vertical and horizontal directions of the image area; wherein the area setting unit is configured to set the contrast evaluation areas with a same shape according to the shape of the image area, and wherein the area setting unit is configured to set the contrast evaluation areas side by side.