Disclosed is an automatic speed changing device in which speed is automatically changed according to a predetermined amount of rotation without user's arbitrary manipulations. The automatic speed changing device of the present invention comprises a hub unit including a hub shaft formed with a sun gear, and a hub shell surrounding the sun gear and having a first ratchet ring and an inner gear formed on an inner peripheral surface of the hub shell; a first travel unit including a driver for rotating the hub shell while closing the other side of the hub shell, and a driven sprocket for rotating the driver by receiving power from a driving sprocket; a second travel unit including a first carrier disposed within the hub shell and mounted at one side thereof with a plurality of planetary gears engaged with the sun gear and the inner gear to rapidly rotate the hub shell, and a second carrier extending from the other side of the first carrier to the driver; and a conversion unit including a second ratchet ring interposed between the driver and the second carrier to transmit rotational force from the rotating driver to the second carrier and the planetary gears of the first carrier, and a conversion spring interposed between the first and second carriers to return the second ratchet ring when pedals of a bicycle are not operated and the bicycle is stopped.