1. A label-information processing device for a multi-viewpoint image including images based on a plurality of image signals obtained by photographing a specific subject from a plurality of viewpoints, the device assigning label information to a remarkable portion depicted in and common to the images, the device comprising: a display unit for displaying a specific image based on a specific image signal of the plurality of the image signals, the specific image signal depicting the remarkable portion in the specific image; and a controller for assigning the label information to the remarkable portion depicted in the specific image based on the specific image signal, the specific image being displayed on the display unit, wherein the controller has: a first function of acquiring coordinates, in a three-dimensional space, of the label information in a coordinate space, the coordinates being in the coordinate space used to express the subject in the three-dimensional space; a second function of displaying, on the display unit, an image based on an image signal of the image signals, the image having a viewpoint different from the viewpoint of the specific image based on the specific image signal, the image signal depicting a remarkable portion identical to the remarkable portion depicted by the specific image signal; a third function of transforming the coordinates, acquired by the first function, of the label information into coordinates, on a plane, at which the label information is assigned to each of the image signals, the images based on the image signals being displayed on the display unit by the second function, the images having the different viewpoints from each other; and a fourth function of assigning the label information to each of the image signals, having the different viewpoints from each other, at predetermined positions obtained from the coordinates transformed by the third function.