To reduce labor of an occupant riding a bicycle with a transmission mechanism under heavy load at the time of ascending a slope, carrying a heavy article, or the like, while there are three kinds of bicycles without a transmission, with a manual transmission mechanism, and with an electric transmission mechanism now.

The speed of this automatic gear changing bicycle can be automatically changed by a simple mechanism. A single crank arm of the bicycle is a joint structure, and a hollow bore is provided, in which a control wire is inserted. One end is fixed to a pedal shaft, and the other end is connected to a normal transmission mechanism. This wire is always pulled to the transmission mechanism by the spring. When a load increases, the transmission mechanism is automatically actuated, so as to reduce the load.