In the medical literature, the ultrasound diagnosis is a quite well user-friendly method due to its non-invasiveness, portability and real-time imaging capabilities. Recently, some fatigue issues (including musculoskeletal injuries) with sonographers have been reported. Most of those issues are mainly due to the physical stress on the sonographers during their considerably long time task. During the diagnosis, the sonographers usually hold their wrist without touching the patients. Moreover, they are sometimes required to apply a force for the probe to push abdomen of the patients in order to obtain clearer ultrasound image. Therefore, our research aims in developing a probe supporting robot to reduce the fatigue of sonographers during the abdominal diagnosis. In this paper, we describe the design of the Waseda-Tokyo Women's Medical-Aloka System No. 2 (WTA-2) which is composed of an ultrasound diagnosis system, an actuated 3-DOFs robot arm with two different interchangeable end-effectors. One is for power assist of applied force, and the other is for supporting sonographer's wrist. The design of the robot, a force sensing method using photo sensors and the control of the robot are detailed. A set of experiments were carried out in order to verify the effectiveness of the proposed system to support sonographers while performing the diagnosis.