a bicycle antitheft device includes an antitheft mechanism switchable between an antitheft state and a release state , wherein the antitheft mechanism includes a first member that moves relative to a second member to move the bicycle forward and backward . a movement controlling mechanism hinders the first member from moving relative to the second member when the antitheft mechanism is in the antitheft state , and a selection mechanism is provided for selecting one of the antitheft state and the release state . alternatively , the antitheft mechanism may include a sound generator for generating a sound when the first member moves relative to the second member and the antitheft mechanism is in the antitheft state . the movement controlling mechanism and the sound generator may be combined into a single antitheft mechanism . the movement controlling mechanism and / or sound generator may be installed inside of an internal transmission such as a hub or crank transmission , or they could be installed inside a handlebar control for the transmission .