PURPOSE: To prevent any damage to an elastic body, being installed between a floating gear and a holder, etc., due to excessive torque transmission at the time of very low speed.

CONSTITUTION: At the time of normal speed of more than idling speed, a pin 14 separates an engaged hole of a first balance shaft 3 against energizing force of a coil spring 15 by dint of centrifugal force working on the pin itself. Therefore, torque transferred to a floating gear from a crank gear is transmitted to a first balance shaft 3 via an elastic body. At the time of very low speed of less than idling speed, the pin 14 is energized by the coil spring 15 and thereby it is engaged with the engaged hole 13 of the first balance shaft 3. The floating gear 6 and the first balance shaft 3 atr solidly connected to each other whereby the torque transmitted to the floating gear 6 from the crank gear is transmitted to the first balance shaft 3 via the pin 14.