PURPOSE: To adapt a machine body for efficient turning near a spin turn as restraining the earth from being roughened unnecessarily by providing a forwarding and reversing mechanism devised to switch over crawler type travellers on the right and the left hand with a speed difference to be driven to directions different with each other.

CONSTITUTION: In a forwarding and reversing mechanism 12 adapted for operational switching, reduction gears 16, provided with gear parts 16a to be engaged and rotated with the fourth gears 10B fixed on the third transmission shaft 10 and gear parts 16b to be engaged and rotated with steering gears 11B in the off-state of a steering clutch, are provided on a reversing transmission shaft 15 stretched in parallel with a steering shaft 11 to be freely slided along the axis of the transmission shaft 15. Steering gears 11B are engaged with reduction gears 16 for operating crawler type travellers 4 and 4 on the right and the left hand so that the speed of the reversing crawler traveller 4 may be smaller than that of the forwarding one.