Scanning lines immediately above the blood vessel are detected using received signals of reflected waves obtained when ultrasonic waves transmitted to the blood vessel are reflected from the blood vessel, and candidates at depth positions that seem to be front and rear walls of the blood vessel are detected based on the received signals of the scanning lines. Then, vascular front and rear walls pairs of front and rear walls are narrowed down from the candidates, and the narrowed-down vascular front and rear walls pair is regarded as one blood vessel and artery/vein identification is performed for each blood vessel. Measurement of vascular function information is performed for the blood vessel determined to be an artery.
