Modifications of multirotor unmanned aerial vehicles are increasingly popular by integrating tools, sensors, or a robotic arm depending on types of applications. In these circumstances, precise motion control against model uncertainty and disturbance is essential. This brief presents an inner-loop control structure to recover the dynamics of a multirotor combined with additional objects similar to the bare multirotor using disturbance observer (DOB)-based approach. In addition, in the process of the inner-loop controller design and the associated stability proof, it is revealed that the safe operation bounds of the modified multirotor in accordance with the amount of inertia added to the multirotor. The proposed control design is validated with flight experiments using a multirotor integrated with a four-DOF robotic arm, where the multirotor is commanded to follow a desired trajectory while the robotic arm is in motion. The results show superior tracking performance of the proposed structure in comparison with backstepping control without DOB.