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 transmitting section that transmits a driving signal for generating the ultrasound transmitted to the subject; a receiving section that receives the ultrasound signal generated from the ultrasound reflected by the subject; a displacement calculating section that measures displacement of the subject based on the ultrasound signal received by the receiving section; an elastic-image generating section that generates an elastic image based on the displacement measured by the displacement calculating section; a storing section that stores one or more of the elastic images generated by the elastic-image generating section; a determination-region setting section that sets, according to a size of an ROI, which is a region of interest, a proper-image determination region for determining a proper image out of the one or more elastic images stored in the storing section; a characteristic calculating section that calculates a region characteristic of the proper-image determination region; and a proper-image determining section that determines based on the region characteristic whether the elastic image in which the proper-image determination region is set is the proper image.