PURPOSE: To perform forward and reverse revolutions by means of a single flywheel and to simplify the constitution of a driving power transmitter, by setting the rotary fulcrum of a pulley lever so as to secure the same belt attachment length between forward and reverse revolutions.
CONSTITUTION: A belt pulley lever 49 containing a belt pulley 51 is supported by a rotary shaft 50, and a tension spring 58 and a drive switching rod 42 engage an arm and the other one of the lever 49 respectively. While a lever 59 containing an idler 61 is supported by a shaft 60 of the rod 42. The shaft 50 has the same belt length between the forward and backward revolutions. The rod 42 is moved left in a forward mode. Then pulleys 51 and 48 are pressed to each other and gears 45 and 35 engage with each other to be driven forward. While the rod 42 is shifted right in a reverse mode and the pulley 51 is pressed to the pulley 48 via an idler pulley 61. Then gears 45 and 36 engages with each other and are driven reverse. Thus it is possible to perform both forward and reverse revolutions by means of a flywheel 46 and to simplify the constitution of a driving power transmitter.