A tractor includes a sensitivity adjusting mechanism 23. The sensitivity adjusting mechanism 23 adjusts operational sensitivity of a spool 17A in a draft control, through changing of a gap S between a pivot arm 30 of a first link mechanism 19 and an operational arm 39 of a second link mechanism 22 by changing a position of a pivot support shaft 42 of a relay arm 41 provided in the second link mechanism 22 in response to an operation on a sensitivity adjusting lever 44 disposed in a driving section.
