PURPOSE: To increase a steering angle in a front wheel accelerating drive condition, and also prevent the increase of danger due to large steering operation in road traveling etc., by providing a checking member, variously limiting the maximum steering angle of a front wheel, so as to be able to be switched in a check-operation condition and a check-released condition.

CONSTITUTION: In a farm working tractor, a metal-made checking arm 44 is provided on both the left and right end parts of a front axle case 24 so as to be oscillatingly and around an axis core P, as a means limitting the maximum steering angle of front wheels 1 and 1. Here with the checking arm 44, oscillation, around the king pin axis X1 of a front wheel supporting case 45 with the steering operation of the front wheel 1, is made possible to be oscillation-switched to a check-operation attitude in which the oscillation is checked by the contact of the freely moving end part 44a of the checking arm 44 and the front wheel supporting case 45, and to a check-release attitude in which the checking is released. Also the checking arm 44 is made to be linked to a manual lever 47, for making shift fork manual operation, via a push/pull wire 48, and is made to switch the checking arm 44 in the check release attitude when the manual lever 47 is operated on a front wheel acceleration-on side.