1. A contour image generating device, comprising: a contour extracting unit that performs one of contour extraction processes that are repeatedly performed by a contour extracting model to a two-dimensional reconstruction image of a subject obtained through detection of radiation emitted from the subject to extract a new second shape of a contour from a first shape of the contour set in advance; a varied area calculating unit that calculates a varied area between the first shape of the contour and the second shape of the contour; a pixel value sum calculating unit that calculates a sum of pixel values of pixels contained in the varied area; a change rate calculating unit that calculates a rate of change that corresponds to a ratio of the varied area and the sum; a completion determining unit that determines whether or not the rate of change is smaller than a threshold set in advance; and a controller that performs control to set the second shape of contour as the first shape of contour and to operate the contour extracting unit, the varied area calculating unit, the pixel value sum calculating unit, the change rate calculating unit, and the completion determining unit again when the completion determining unit determines that the rate of change is larger than the threshold, and that performs control to output two-dimensional contour image each containing the second shape of the contour when the completion determining unit determines that the rate of change is smaller than the threshold.