A set of feature points including three feature points in a B-mode image acquired by an ultrasonic measurement are evaluated on a criterion as to whether a position of a circle passing through the three feature points is regarded as a position of a vessel wall. As evaluation items, there are the number of feature points located on a contour of the circle passing through the three feature points, position changes of the feature points with time, luminance of the feature points, the number of feature points inside the circle, etc. As a result of the evaluation, if a predetermined condition is satisfied, the position of the vessel wall is determined to be in the position of the circle passing through the selected three feature points, and the vessel position is detected.
