A gear-shifting mechanism driving device for allowing a speedy and accurate gear-shifting operation has a light weight even in the case where a shift spindle is short. The gear-shifting mechanism driving device includes a transmission for selectively establishing a gear combination by the turning of a shift spindle for turning a master arm, and an actuator as a power source for turning the shift spindle, in which the gear-shifting mechanism driving device includes a reduction gear shaft provided in a power transmission path between the actuator and the shift spindle, a first gear provided on one end side of the reduction gear shaft to receive power from the actuator side, and a second gear provided on another end side of the reduction gear shaft to transmit power to the shift spindle side. The reduction gear shaft is made of a torsion bar.