1. A medical imaging system, comprising: an image generating unit which captures a front chest image of a subject and generates a front chest medical image which is a still image; a region extracting unit which extracts a lung field region from the front chest medical image, divides the lung field region into a plurality of small regions, extracts high spatial frequency components with a high-pass filter for each of the plurality of small regions of the lung field region, calculates a maximum value or an integral value of response of the extracted high spatial frequency components in a range between Nyquist frequency fN and (fN−Δf), wherein Δf represents a predetermined value, compares either the calculated maximum value or the calculated integral value to a threshold value predetermined for the maximum value or to a threshold value predetermined for the integral value, and extracts, from the plurality of small regions, small regions having the threshold value or smaller as a plurality of local regions which include no edge of a rib bone; a motion judging unit which extracts high spatial frequency components from each of the plurality of local regions extracted by the region extracting unit, calculates a motion feature value based on the extracted high spatial frequency components of the extracted plurality of local regions, and judges whether there is any motion in the subject during image capture based on the calculated motion feature value; and a controlling unit which causes an outputting unit to output a judgment result made by the motion judging unit.