The invention detects a change in a subject by detecting the subject from an image, acquiring a feature quantity distribution that indicates shape information of the detected subject, accumulating the shape information that is indicated by the acquired feature quantity distribution and comparing the shape information a predetermined period of time before with the current shape information by using the accumulated shape information. Here, the invention acquires the feature quantity distribution of the subject from a processing target area extracted from an image area that includes the subject. The invention detects a change in the subject by using accumulated shape change information acquired from the shape information. The invention detects a change in the subject by using averaged shape change information obtained by averaging the shape change information.