1. An ultrasound observation apparatus that generates an ultrasound image based on an ultrasound signal generated from ultrasound transmitted to a subject and reflected, the ultrasound observation apparatus comprising: a computer configured to: transmit a driving signal for generating the ultrasound transmitted to the subject; receive the ultrasound signal generated from the ultrasound reflected by the subject; measure displacement of the subject based on the ultrasound signal received; generate one or more elastic images based on the displacement measured; control a memory to store the one or more elastic images generated; compare a size of a region of interest (ROI) in an elastic image of the one or more elastic images, with a threshold; in a first case where the size of the ROI is equal to or smaller than the threshold, set an entirety of the ROI as a proper-image determination region; in a second case where the size of the ROI is larger than the threshold, set one or more partial regions, where each of the one or more partial regions is a part of the ROI, as the proper-image determination region, wherein the proper-image determination region is a region for determining a proper image suitable for diagnosis and storage of the proper image out of the one or more elastic images stored in the memory; calculate a region characteristic of the proper-image determination region based on the displacement measured or on elasticity information of the elastic images generated; and determine based on the region characteristic calculated whether the elastic image in which the proper-image determination region is set is the proper image or not.