Nonlinear Stabilization of the Rotational Inverted Pendulum using the Controlled Lagrangians Method （制御ラグランジアン法による回転型倒立振子の非線形安定化制御） Control systems that have more degrees of freedom than the number of actuators are called underactuated systems. The method of controlled Lagrangians has been developed as a technique to stabilize underactuated Lagrangian mechanical systems with symmetry. The basic idea of this method is to modify the kinetic energy and the potential energy to produce a new Lagrangian which describes closed-loop dynamics. This method has the advantage that stabilization can be understood in terms of energy-based Lyapunov functions which give information on how to design control inputs to achieve closed-loop stability. The rotational inverted pendulum, which has two rotational degrees of freedom and one actuator, is an example of an underactuated system. The main purpose of this paper is to stabilize the rotational inverted pendulum at the straight up state using the method of controlled Lagrangians. The control input designed by the present method provides Lyapunov stability. Further another controller which provides asymptotic stability to the inverted pendulum at the straight up state is designed as well.