A blood vessel ultrasonic image measuring method capable of facilitating the positioning of an ultrasonic probe and acquiring sufficient positioning accuracy is provided. Because of inclusion of an around-X-axis positioning step of causing a multiaxis driving device 26 to position an ultrasonic probe such that distances from respective ultrasonic array probes to the center of a blood vessel are equalized, and an X-axis direction positioning step and an around-Z-axis positioning step of causing the multiaxis driving device to position the ultrasonic probe such that the image of the blood vessel is positioned at the center portion in the width direction of the first short axis image display area G1 and the second short axis image display area G2, the positioning may be performed by using the positions in the longitudinal direction of the ultrasonic array probes relative to the blood vessel or the distances of the ultrasonic array probes to the blood vessel and, therefore, the ultrasonic probe may simply and easily be positioned on the blood vessel of a living body with higher accuracy.