1. A medical image diagnosis assisting apparatus, comprising: a lung field area extraction means for extracting a lung field area from a three-dimensional medical image representing a chest region of a subject; a branch structure area extraction means for extracting a branch structure area from the three-dimensional medical image; a lung field local sub-area estimation means for dividing a branch structure local area representing a portion of the branch structure area into a plurality of branch structure local sub-areas and estimating a lung field local sub-area in the lung field area functionally associated with each divided branch structure local sub-area based on the branch structure area; a pulmonary evaluation value obtaining means for obtaining a pulmonary evaluation value of pulmonary function and/or morphology in each estimated lung field local sub-area; and a display control means for displaying, in a morphological image representing morphology of at least a portion of the branch structure local area generated from the three-dimensional medical image, the pulmonary evaluation value in each lung field local sub-area functionally associated with each branch structure local sub-area in the morphological image superimposed such that a correspondence relationship between the pulmonary evaluation value and the branch structure local sub-area in the morphological image is visually recognizable, wherein the display control means displays the pulmonary evaluation value as a color map, the color map including map elements, colors and density values of the map elements being allocated according to the pulmonary evaluation value of each of the branch structure local sub-areas, each boundary position between the map elements corresponding to each boundary position between the branch structure local sub-areas.