In minimally invasive robotic surgeries, insertion ports become the fulcrum of surgical instruments such as forceps. Misalignment between the insertion port and the remote center of motion (RCM) of the robotic arm or external forces on the tip of its forceps generate undesirable forces and friction between the port and the forceps. It not only damages the port but also worsens position control performance. In this study, we developed a pneumatic surgical robotic arm that does not require the positioning of its RCM on the insertion port. Force-compensatable joints that are compliant with the joint position and have the ability to output torque are employed. We call these joints “semi-active joints”, and we realize them with pneumatic actuators with high backdrivability. The posture of the robotic arm with the semi-active joints is decided passively by the constraint of the insertion port. In addition, the robotic arm can compensate for the weight of its forceps and external forces to reduce the load on the insertion port. With the prototype surgical robotic arm system, the position control of the robotic forceps was performed, and we evaluated the accuracy of the arm’s movement and the forces exerted on the insertion port. The experimental results showed that the load on the insertion port was decreased by 74%, when using semi-active joints.