An auto transmission device for a bicycle is provided to prevent loosening of a connecting member due to a size change of a driving pulley and a driven pulley by allowing belt tension rollers to pull upper and lower parts of the connecting member by third spring. An auto transmission device for a bicycle includes a first rotary plate(110), a synchronizing gear(200), a driving pulley(120), a driven pulley, and a connecting member(140). The first rotary plate has a driving shaft(100) at a center. The driving shaft is rotated by a pedal. The synchronizing gear is installed on an outer peripheral surface of the driving shaft. The driving pulley is engaged with a circumference of the synchronizing gear and has pulley pieces(300) with first pulley rotary shafts(400) connected to the first rotary plate. The driven pulley is installed on a rear wheel of the bicycle. The connecting member connects the driving pulley and the driven pulley. When the first rotary plate is applied with large force, the pulley pieces are rotated around the first pulley rotary shaft in a direction in which a size of the driving pulley is reduced while a size of the driven pulley is increased.
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