A derailleur includes a fixed component, a movable component, a linkage component, a deceleration component and a drive component. The fixed component is for being fixed to a bicycle frame. The movable component is for being fixed to a chain guide. The linkage component includes a first linkage and a second linkage. Two ends of the first linkage pivot on the fixed component and the movable component, respectively. Two ends of the second linkage pivot on the fixed component and the movable component, respectively. The deceleration component includes a circular spline, a flex spline and a wave generator. The flex spline is disposed inside the circular spline, for rotating the first linkage and the second linkage relative to the fixed component. The wave generator is disposed inside the flex spline. The drive component is connected to the wave generator, for rotating the wave generator.