An apparatus for measuring a short-axis image of a blood vessel under a skin of a living being, the apparatus including an ultrasonic probe including an ultrasonic array which has a plurality of ultrasonic transducers arranged in one direction in an emission surface and which emits, from the emission surface, an ultrasonic beam toward the blood vessel, the ultrasonic probe additionally including a main frame which is adapted to be placed on the skin of the living being, an x-axis supporting device which is supported by the main frame and which supports the ultrasonic array such that the ultrasonic array is rotatable about an x axis parallel to the direction of arrangement of the ultrasonic transducers in the emission surface, and an x-axis control device which controls a posture of the ultrasonic array supported by the x-axis supporting device such that in a y-z plane, the emission surface of the ultrasonic array is parallel to the blood vessel.