Thrust generated by limbs is quite important in swimming. However, the fluid force acting on limbs during unsteady motion such as swimming has been difficult to measure. Therefore, the authors developed an underwater robot arm, which can perform 5 degrees-of-motion, and can represent both upper and lower limb’s swimming motion. The contents of this paper consist of the specification of the robot arm, an experiment to measure the unsteady fluid force using the robot arm, and the present status of the fluid force modeling based on the experimental results. As the result, it was verified that the unsteady fluid force during the swimming motion can be measured by the experimental setup. In addition, the validity of the fluid force model which was identified based on the experimental results was also verified.