A first blood vessel diameter and a second blood vessel diameter are measured using a first ultrasonic probe and a second ultrasonic probe that are provided close to a blood vessel of a subject so as to be situated at a given distance (Lp). Characteristic phases of a pulse wave are determined from the peak of a second-order differential value of the first blood vessel diameter and the peak of a second-order differential value of the second blood vessel diameter, and the difference (Δt) in pulse wave transit time is calculated from the difference in timing between the characteristic phases to calculate pulse wave velocity (PWV). A given calculation process that uses the pulse wave velocity (PWV) and the measured blood vessel diameter as variables is performed to calculate blood pressure.
