PURPOSE: To enable accurate determination of an elastic modulus of a blood vessel by measuring the diameter of the blood vessel at wherever position an ultrasonic probe is in the blood vessel.

CONSTITUTION: A distance calculating section 23 calculates a distance to the wall of the blood vessel (internal and external walls) from the center of the ultrasonic probe based on the time taken until the reflected wave of an ultrasonic beam is received after being transmitted from ultrasonic vibrators 14 provided at three points of the outer circumference of the tip of an ultrasonic probe, and a blood vessel diameter calculating section 24 calculates the diameter of the blood vessel or the like using the since formula from the distance calculated by the distance calculating section 23. A blood vessel elastic modulus calculating section 27 calculates an elastic modulus of the blood vessel based on a pulse pressure calculated by a pulse pressure calculating means 25, an upstream diameter of the blood vessel and the like.