A prototype of a 7 m diameter Butterfly Wind Turbine equipped with a new over-speed control system using movable arms was developed. A unit consisting of a movable arm (MA) and an aileron installed below the MA is inclined according to the balance of the centrifugal force, the aerodynamic force, and the gravitational force acting on the unit. The slant angle of a movable arm (and an aileron) was measured to show the behavior. It was clarified that the slant angle amplitude was different between the state where the aerodynamic force was dominant and the state where the centrifugal force was dominant. In the aerodynamic dominant state, the azimuth angle in which the slant angle took the maximum depended on the wind speed level. The measured maximum rotation speed of the wind turbine is currently less than the design maximum rotation speed, and the operation is as expected.