1. An ultrasound diagnostic apparatus which determines a type of target tissue inside a body of a subject, the ultrasound diagnostic apparatus comprising: an image forming unit configured to form an ultrasound image which corresponds to an echo signal received from the subject to which a contrast medium has been administered; a region of interest setting unit configured to set, in a target region corresponding to the target tissue on the ultrasound image formed by the image forming unit, a first region of interest and a second region of interest which are two regions different from each other; a feature value extraction unit configured to extract, as a feature value, a difference in brightness between the first region of interest and the second region of interest set by the region of interest setting unit; and a type determination unit configured to determine, based on the feature value extracted by the feature value extraction unit, the type of the target tissue.