To provide an index which accurately indicates a level of invasion of cancer into a bone region.SOLUTION : An image processing device executes a program to obtain an index with respect to a bony metastasis on the basis of a three-dimensional nuclear medicine image such as a SPECT image. The image processing device comprises : an image data acquisition section 10 where data on the SPECT image and a CT image of a subject is input; a bone region extraction section 20 which extracts a bone regions in the CT image; a bone region identification section 21 which identifies the bone region in the SPECT image corresponding to the bone region in the CT image; a normalizing section 22 which normalizes a pixel value of the bone region in the SPECT image; a bony metastasis region extraction section 23 which obtains a bony metastasis region having the pixel value equal to or larger than a predetermined threshold value in the bone region in the normalized SPECT image; and an index calculation section 24 which calculates the index with respect to a bony metastasis on the basis of volumes of the bone region and bone metastasis region.SELECTED DRAWING : Figure 1