This paper provides a robotic ultrasound transducer positioning system for minimally invasive ultrasonic therapy by manipulating microbubbles in blood vessel, aiming at effective high-intensity focused ultrasound (HIFU) and acoustic drug/gene delivery. To realize such a technique in vivo, accurate and dynamic positioning of an ultrasound transducer is required. For this purpose, we have recently developed a robotic system with a parallel link structure. The novel system consists of the newly developed robot with an ultrasound transducer, an optical tracking device, and an ultrasound imaging device. As an interface for robot control, we have also developed a planning software to plan target position and direction on an echogram. The optical tracking device is used to integrate the robot and the echogram coordinates. To validate the system feasibility, this study evaluated the system performance by conducting experiments of microbubble manipulation in an artificial blood vessel.