A contour extracting unit of a contour image generating device performs one-time contour extraction to a two-dimensional reconstruction image by a contour extracting model so as to extract a second shape of a contour from a first shape of a contour. A completion determining unit determines whether or not a rate of change is smaller than a threshold. The rate of change is a ratio of a varied area and a sum of pixel values of pixels in the varied area, the varied area being between the first shape of the contour and the second shape of the contour. When the rate of change is large, a repetitive controller performs control to operate the contour extracting unit and the like again. When the rate of change is small, the repetitive controller performs control to output a two-dimensional contour image containing the second shape of the contour.