Internal differential transmission for bicycle

The invention provides an internal differential transmission for a bicycle, which comprises a driving body used for transmitting manual driving force, a fixing piece used for fixing a speed changing unit, the speed changing unit and a control part. The speed changing unit changes the speed of motion input by the driving body and then outputs the motion through the second rotating body or the fourth rotating body. The control part can move along the axial direction of the first fixed shaft, and can control the connection and separation of the first rotating body and the second rotating body with the fixing piece and the connection and separation of the first rotating body and the fourth rotating body with the driving body in the moving process. The differential gear train composed of the first rotating body, the second rotating body, the third rotating body and the fourth rotating body serves as a main body, the gear modulus can be increased while the boundary dimension is met, the bending strength is greatly