1. An image generation apparatus comprising: classification means for classifying a plurality of segments, obtained by dividing a medical image generated by radiography of a chest of a subject and including structures having anatomic characteristics into a plurality of types of segments in such a manner that segments that include structures having a similar anatomic characteristic are classified into the same type of segments; and artificial image generation means for generating an artificial image of at least one of the plurality of segments, the artificial image including a normal structure and being generated based on the type of the at least one of the plurality of segments, wherein the classification means includes: lung field recognition means for recognizing lung fields in the chest image; and rib recognition means for recognizing ribs in the chest image, and wherein the classification means is configured to classify each of the plurality of segments into one of the plurality of types based on a position of each of the plurality of segments in a lung field, wherein said classification means further determines a number of ribs overlapping each of the plurality of segments, and the classification means further classifies the segments based on the number of ribs overlapping each segment.