[PROBLEMS] to improve the stability of the bone sinti our automatic normalization of the image. [MEANS FOR SOLVING PROBLEMS] embodiment is suitable, sinti bone to create a pixel value histogram of the image data representing the image Photography; To set a plurality of threshold values ​​to the pixel value based on the pixel value histogram; To calculate the average pixel value for each of a plurality of threshold values ​​by the setting; The value of the calculated pixel value to the average value lists from the largest; And based on at least a portion of a set of the pixel value the average value is listed in the sequence, the image to determine the reference value for normalizing the data; including, and to determine the reference value, the pixel value the average value is listed in the order is the pixel value from the set of average values ​​to determine a straight line approximation of a small area; And to calculate the reference value based on the straight line; includes.