Up to now, there are different kinds of robot-assisted ultrasound diagnostic systems proposed in the last decade. However, the compensation of the ultrasound probe position according to the patient movement is still one of the most important and useful functions required for those systems. For this purpose, in this research, we aim at developing an automated diagnostic system for the measurement of the wave intensity which is usually measured at the common carotid artery. In particular, in this paper, we focus on proposing a robust visual servoing method for tracking out-of-plane motion for a robot-assisted medical ultrasound diagnostic system by using a conventional 2D probe. A robotic device which manipulates the ultrasound probe firstly scans a small area around the target position and records several B-mode images at a regular interval. In order to track the out-of-plane motion, an inter-frame block matching method has been proposed and implemented on the Waseda-Tokyo Women's Medical-Aloka Blood Flow Measurement System No. 2 Refined (WTA-2R). A set of experiments was proposed to verify the effectiveness of the proposed method. From the experimental results, we could confirm its robustness while doing the task with real human tissues.