A rear derailleur is adapted to be secured to a side of a rear fork portion of a vehicle frame. When the front derailleur pull tight or release a transmission cable, a transmission lever is move back and forth in a threaded shaft base and a foot portion of a lever control cable also pulls a chain retaining lever to rotate, so that a position of a distal point of the rotating chain retaining lever resulting from a movement of a retracing spring is changed. Accordingly, effects such as gear shift-up, gear fixing, gear shift-down, and angle adjusting of chain retaining steering wheel are achieved. In addition, occurrence of disorder of the derailleur system and slipping off of the chain are reduced.