[Problem to be Solved] To improve stability of automatic normalization of a bone scintigraphy image.[Solution] A preferred embodiment includes: creating a pixel value histogram of image data representing a bone scintigraphy image; setting a plurality of thresholds related to pixel values based on the pixel value histogram; calculating respective average pixel values for the set thresholds; arranging the calculated average pixel values in order from the largest value; and determining a reference value for normalizing the image data based on at least part of a set of the average pixel values arranged in the order. The determining the reference value includes: determining one straight line that approximates a region of small average pixel values out of the set of the average pixel values arranged in the order; and calculating the reference value based on the straight line.