PURPOSE: To prevent the slip and sink of rear wheels by providing a CPU with a controlling part outputting a command to lift a lift arm with regular time intervals when a longitudinal inclination sensor detects that the machine is in top-lifted state relative to the average state.

CONSTITUTION: Detection signals of a longitudinal inclination sensor S3 are averaged over a relatively long period. The present longitudinal inclination of a tractor 1 is detected or a short-time average value of the inclination is calculated to judge whether the tractor is top-lifted state or not. If the tractor is judged to be in the top-lifted state, a judgement is passed on whether the timer for lifting a lift arm 3 for a prescribed time is in the state of ON or not. If the timer is judged to be in the state of ON, a proportional solenoid valve for lifting is operated by the operation command transmitted from a control part to lift the lift arm 3. The sink and slip of the rear wheels of the tractor and the lowering of the pulling force of the tractor can be prevented.