1. A distance estimation device comprising: a first focal-depth image obtaining unit configured to obtain a first focal-depth image focused at a first focal depth; a second focal-depth image obtaining unit configured to obtain a second focal-depth image focused at a second focal depth different from the first focal depth; an all-in-focus image generation unit configured to generate an all-in-focus image focused in a focal range wider than each of a focal range of the first focal-depth image and a focal range of the second focal-depth image; a first generation unit configured to (i) set a plurality of focal depths within the focal range of the all-in-focus image, (ii) generate, from the all-in-focus image, a plurality of blurred images which are focused at the first focal depth and blurred at the plurality of focal depths, (iii) select, for each of image regions included in the first focal-depth image, a blurred image having a sub-image that is most similar to a sub-image in the image region, from among the blurred images at the focal depths, and (iv) generate a first distance map indicating the focal depth of the selected blurred image as a focal distance of the image region; a second generation unit configured to (i) generate, from the all-in-focus image, a plurality of blurred images which are focused at the second focal depth and blurred at the plurality of focal depths, (ii) select, for each of image regions included in the second focal-depth image, a blurred image having a sub-image that is most similar to a sub-image in the image region, from among the blurred images at the focal depths, and (iii) generate a second distance map indicating the focal depth of the selected blurred image as a focal distance of the image region; and a distance map synthesis unit configured to synthesize the generated first distance map and the generated second distance map to generate a synthetic distance map.