1. A contour image generating device, comprising: one or more processors, configured to: perform 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; calculate a varied area between the first shape of the contour and the second shape of the contour; calculate a sum of pixel values of pixels contained in the varied area; calculate a rate of change that corresponds to a ratio of the varied area and the sum; determine whether or not the rate of change is smaller than a threshold set in advance; and perform control to set the second shape of contour as the first shape of contour and to perform control so that the performance of one of contour extraction processes is repeated when the rate of change is determined to be larger than the threshold, and perform control to output two-dimensional contour image each containing the second shape of the contour when the rate of change is determined to be smaller than the threshold.