1. A medical report generation apparatus comprising: a key image selection unit that selects, as a key image representing a diagnostic characteristic feature, a slice image generated based on three-dimensional medical image data obtained by imaging a subject; a selected position input unit that accepts a position input by a user on the key image and that inputs the accepted position as a selected position in the key image; an abnormal shadow extraction unit that extracts an abnormal shadow within a predetermined neighborhood range from the selected position; a supplementary image determination unit that moves the selected position into the extracted abnormal shadow based on the key image and the selected position in case that the abnormal shadow is extracted and that determines a slice image that includes the moved selected position and represents a cross section different from a cross section represented by the key image, as a supplementary image in the three-dimensional medical image data; and a medical report generation unit that generates a medical report including the key image and information for identifying the supplementary image.