1. A thoracic diagnosis assistance system, comprising: an imaging unit which images chest portion movements in at least one breathing cycle and which generates a plurality of successive image frames; an extraction unit which extracts a lung field region from each of the plurality of image frames generated by the imaging unit; a region dividing unit which divides the lung field region extracted by the extraction unit into a plurality of sub-regions and which correlates the sub-regions among the plurality of image frames; an analysis unit which performs an analysis for each of the sub-regions correlated in the plurality of image frames to calculate an inspiratory feature quantity and an expiratory feature quantity of the sub-region and to calculate a value of a ratio of the calculated inspiratory feature quantity to expiratory feature quantity and which creates a histogram of the calculated ratio value; and a display unit which displays the histogram created by the analysis unit.