1. An image processing device, comprising: an obtaining unit which successively obtains a plurality of images; a synthesis unit which partially superimposes the plurality of images obtained by the obtaining unit to generate a synthesis image when the obtaining unit obtains the images; a first judgment unit which judges whether the synthesis image contains a feature region having a predetermined feature when the synthesis image is generated by the synthesis unit; and a control unit which makes predetermined processing be executed when the first judgment unit judges that the synthesis image contains the feature region.