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; and an analysis unit which performs an analysis for each of the sub-regions correlated in the plurality of image frames to calculate an inspiratory period feature quantity and an expiratory period feature quantity of the sub-region and to calculate an index value based on the calculated inspiratory period feature quantity and expiratory period feature quantity, wherein the inspiratory period feature quantity is a value based on inter-frame difference values obtained by calculating a difference in pixel signal values between the image frames chronologically adjacent among a group of the image frames corresponding to an inspiratory period, and the expiratory period feature quantity is a value based on inter-frame difference values obtained by calculating a difference in pixel signal values between the image frames chronologically adjacent among a group of the image frames corresponding to an expiratory period.