To provide a bicycle provided with an auxiliary transmission gear with which a rear wheel can receive turning force larger than that of a rear wheel gear and efficiently transmit the turning force of a pedal to the rear wheel to reinforce the driving force of the bicycle.

This bicycle is provided with a crankshaft pivotally provided on a body frame vertically movably and fixed to a pedal gear, and an auxiliary transmission gear. The auxiliary transmission gear with one end pivotally supported by a rear wheel body frame is provided with an auxiliary frame disposed to be vertically movable according to the vertical movement of the crankshaft, an auxiliary gear pivotally provided on the auxiliary frame and meshing with a chain, and a pulley fixed coaxially with the auxiliary gear. The pulley contacts with a rear wheel in accordance with the downward movement of the crankshaft.