PURPOSE: To prevent a tractor from falling down while a lift arm is being stood up and a sliding component is being extended, by a method wherein the lift arm is stood up after touch of a support leg to the ground through an orderly operation controller and next to a completion of the standing the sliding component is made to extend.

CONSTITUTION: To dump a semitrailer 13 of a tractor (A) a support leg 2 is stretched first by an extension of a cylinder 3, and the support leg 2 is made to touch the ground through an extension of a cylinder 5. Then, after standing of a lift arm 7 by an extension of a cylinder 9 for undulation of the lift arm a cylinder 12 for expansion and contraction of a sliding component is extended, a sliding component 10 is extended and the semitrailer 13 is made to dump reliably. Then, to lay down the semitrailer it is carried out by reversing an operation for dumping.