1. An ultrasound diagnostic apparatus for selecting an ultrasound image signal of at least one frame from among ultrasound image signals of a plurality of frames acquired with respect to a subject through an ultrasound probe, and displaying the ultrasound image signal of the frame that is selected on a display, the ultrasound diagnostic apparatus comprising a control circuit including: an ultrasound image acquirer that acquires the ultrasound image signals of the plurality of frames; an evaluation target determiner that analyzes the ultrasound image signal of each of the frames and that determines the frame to be an evaluation target frame when the ultrasound image signal of the frame includes a target image section depicting a joint; a disease progression score calculator that calculates, for each evaluation target frame that is determined, a disease progression score quantifying activity of a disease, the disease progression score calculator calculating the disease progression score using an ultrasound image signal of the target image section included in the ultrasound image signal of the evaluation target frame; a selector that selects, in accordance with a predetermined numerical process, at least one disease progression score from among disease progression scores that are calculated; and a display controller that controls the display to display the disease progression score that is selected and an ultrasound image of a frame for which the disease progression score that is selected is calculated.