To obtain an automatic rotational transmission gear in a wheel mechanism, to effectively use a spiral spring, to create an energy saving, high- performance and space-saving bicycle, and to create a high-performance monocycle.

A gear on a pedal side or a wheel side of a conventional bicycle is replaced with a 'spiral spring gear' which is a spiral spring having a projection for a gear on its back or side to make a manual transmission gear mechanism automatic at the time of traveling. In the mechanism, the spiral spring gear is wound up from inside at the time of braking to effectively utilize energy. The spiral spring gear mechanism is housed in a tire, and a pedal shaft and a rear wheel shaft are identical to save a space of a bicycle. Using this new bicycle pertinent art, a conventional monocycle is provided with a mechanism including the dynamic use of a training wheel.