A control part includes a transmission angle control unit that controls a transmission angle of ultrasonic waves transmitted from three ultrasonic arrays, a first reflected wave measuring unit that measures a first arrival time of a first reflected wave which is reflected by a blood vessel and is first to arrive at the ultrasonic arrays, a second reflected wave measuring unit that measures a second arrival time of a second reflected wave which arrives at the ultrasonic arrays after a predetermined time from the first arrival time, an outside diameter computation unit that computes an outside diameter of the blood vessel based on the first arrival time of three first reflected waves, and an inside diameter of the blood vessel computation unit that computes an inside diameter based on the second arrival time of three second reflected waves.