A driving force transmission mechanism and method according to the present invention transmits the driving force of a single motor selectively to a plurality of driven gears, comprising a sun gear rotated either in a forward or reverse direction by the motor, a planetary gear engaging the sun gear, and a plurality of driven gears arranged along the revolution orbit of the planetary gear. A driven gear with which the planetary gear is desired to engage is selected by rotating the sun gear in one direction. The driving force is transmitted to the selected driven gear via the planetary gear by rotation of the sun gear in the opposite direction. The proper gear is selected by use of a sensor. Using the driving force transmission mechanism, various driving systems can be driven by a single motor without the need for a complex switching mechanism. The transmission mechanism may use more than one planetary gear and may use more than one planetary gear assembly.