1. An image processing apparatus comprising: a corresponding region connecting unit that connects regions that depict the same target between a series of images captured in time series, thereby sets at least one connected region; a connected region feature data calculation unit that calculates feature data of the connected region; a digest index value calculation unit that calculates a digest index value corresponding to a degree at which the target depicted in the series of images is aggregated in each image of the series of images, based on the feature data; and a digest image detector that detects a digest image based on the digest index value.