An image processing device (500) according to the present invention is provided with: a visibility index computation means (501) which, upon receiving two or more images to be merged, computes, for each image to be merged, a visibility index, which is an index relating to the visibility of an object to be viewed in the image, on the basis of a contrast value of the image and the amount of noise included in the image, wherein the two or more images to be merged have different characteristics and have at least an overlap region; a merge ratio computation means (502) which computes an image merge ratio between the images to be merged, on the basis of the computed visibility indices; and an image merging means (503) which merges the images to be merged, on the basis of the computed merge ratio, to generate a merged image.