In this report, the dynamics of the Hula-Hoop whose rotational motion is kept stabilized by reciprocating motion of the waist in horizontal direction are treated. Considering a three dimensional dynamic system with a mathematical pendulum whose supported point was caused to move periodically along the fixed axis and not to move vertically, the horizontal and vertical motions of the pendulum were analyzed in detail. Coupled set of nonlinear ordinary differential equations of the second order which were led by applying Lagrange's equations to the system were solved approximately. The basic mechanism of the stable motion of the pendulum due to an oscillatory motion of the supported point is made clear on account of the consideration for the approximate solution of the equations. The influence of the motion of the supported point on these motions was clarified, while these motions depended closely on the movement of the supported point. In addition, the consideration for the stability of the system led analytically such conclusion as the stable motions were preserved on account of the vertical motion of the Hula-Hoop. To examine the validity of the theory, the experimental model was provided. The experimental results were shown to be in qualitative good agreement with the theoretical ones.