To facilitate connection and disconnection of both locker arms by a switching means by pressing a locker arm for high speed at a first end to a locker shaft, and connecting a second end to the pivotal end of a locker arm for low speed to position the high speed locker arm to the low speed locker arm.

A cam shaft 11 born on a cylinder head is so constructed that a cam 11 for low speed and a cam 12 for high speed are integrally formed, and a locker arm 21 for low speed, the base end of which is pivoted on a locker shaft 20 is pivoted by sliding contact with the cam 11 for low speed. Further, this gear is provided with a locker arm 30 for high speed pivoted by a sliding contact with the cam 12 for high speed, a first end of the locker arm can be pressed to the locker shaft 20, and a second end 32 is connected to a pivotal end 23 of the low speed locker arm 21. The first end 31 is energized to be pressed to the locker shaft 20 by a lost motion mechanism 70, thereby switching the high speed locker arm 30 to a functional state.