Abstract: An amphibian bicycle wherein the central screw portion for the rear wheel hub part coupled with the large circular disc plate, and that for the inner body end coupled with the small circular disc plate have screw portions in a direction contrary to that for the gears, whereby loosening of the gears is prevented in the case of fastening the large circular disc plate integral with the multi-stage gears with the bolts during its forward and backward motions on the water. In the case of operations on land, all the related bolts are separated for the driving operation.In case of operation on water, the air tubes having a plurality of air chambers, semi-circle frames and fixing pipes uniformly distribute and support the weight of the bicycle and the human body.