To position right and left mammograms in an appropriate method corresponding to conditions that mammography is carried out.

According to an image processing device in this invention, when the left mammogram data L and the right mammogram data R acquired by imaging from the same direction on a same patient and auxiliary information are entered, whether the positioning at the time of imaging is good or wrong is determined from the entered auxiliary information. If the positioning at the time of imaging is determined to be good, a positioning algorithm A for positioning the right and left mammograms from a subject region is selected, and the positioning according to the positioning algorithm A is carried out. If the positioning at the time of imaging is determined to be wrong, a positioning algorithm B for positioning the right and left mammograms from mammary glands is selected, and the positioning of the right and left mammograms according to the positioning algorithm B is carried out.