A quasi-optimal control system of a flexible rotating cylindrical arm in water is developed focusing on a linearization of the nonlinear fluid drag force. By computer simulations, it is clear that when the compensation coefficient of fluid drag force is large, the arm approaches rapidly near the target but does not converge to the target. As a result, it is better to use the small compensation coefficient obtained by approximating a fluid drag force coefficient of the cylinder derived with Oseen's approximation. Further, to improve the slow approach of the small compensation coefficient, an adaptive control system is developed. In this system, feedback gains of several arm speeds calculated with the optimal control theory are stored in the controller, and the arm is controlled using the optimal feedback gain selected according to the arm speed. Then, model experiments were carried out and the performance of the developed control systems was confirmed.