1. An ultrasound diagnostic apparatus, comprising: an ultrasound probe which performs transmission and reception of an ultrasonic beam toward a blood vessel in a subject based on an actuation signal and outputs a reception signal of the ultrasonic beam; and a diagnostic apparatus body which is connected to the ultrasound probe and performs a processing based on an operation program, wherein the diagnostic apparatus body comprises: a transmission/reception circuit configured to produce the actuation signal and produce a sound ray signal obtained by processing the reception signal; a candidate vascular wall boundary point determining processor configured to determine candidate vascular wall boundary points based on the sound ray signal; a first evaluation value calculating processor configured to calculate a first evaluation value representing accuracy of each of the determined candidate vascular wall boundary points as a vascular wall boundary point; a second evaluation value calculating processor configured to, by means of a sound ray signal in a predetermined depth range including the determined candidate vascular wall boundary points and a sound ray signal adjacent to the sound ray signal, calculate a second evaluation value representing similarity between the sound ray signal and the adjacent sound ray signal; a third evaluation value calculating processor configured to calculate a third evaluation value for determining the vascular wall boundary point from among the candidate vascular wall boundary points based on the first evaluation value and the second evaluation value; and a boundary determining processor configured to determine the vascular wall boundary point based on the third evaluation value and determine a vascular wall boundary based on the determined vascular wall boundary point.