1. An ultrasound diagnostic apparatus which determines a type of target tissue inside a body of a subject, the ultrasound diagnostic apparatus comprising: an ultrasound probe which transmits ultrasound to the subject; and a processor or integrated circuit which is configured to: form an ultrasound image which corresponds to an echo signal received by the ultrasound probe from the subject to which a contrast medium has been administered; set, in a target region corresponding to the target tissue on the ultrasound image, a first region of interest and a second region of interest which are two regions different from each other, wherein the first region of interest and the second region of interest are set such that the first region of interest surrounds an outer periphery of the second region of interest; extract feature values including a first feature value and a second feature value, the first feature value being a difference in brightness between the first region of interest and the second region of interest within a predetermined first time period, and the second feature value being a difference in brightness between the first region of interest and the second region of interest within a predetermined second time period; and determine, based on at least each of the first feature value and the second feature value, the type of the target tissue.