To provide an automatic transmission of simple structure and a bicycle using the automatic transmission.

This automatic transmission has an input means 27 inputting rotation, deceleration means 28 reducing a speed of the rotation input to the input means 27 by prescribed speed reduction ratio also to be provided with one-way clutch 28a, output means 29 outputting the rotation decelerated by the deceleration means 28, and a connection means 30 connecting the input/ output means 27, 29 without through the deceleration means 28 also generating a slip in accordance with torque of these input/output means 27, 29.