To inhibit the shift of a change lever to a position to accelerate a machine body exceeding the legal limit speed in the case of working on a field while directing a steering wheel opposite to the rotary.

A back restriction piece 28a and a back speed restriction piece 28b are placed in a state gyrating in the movable range of a tilting rod 24 in a freely protrusible and retractable manner. The pieces 28a and 28b are fixed to an L-shaped rod 27 with a phase difference. In the case of performing a field work while moving the worker backward by directing a steering wheel toward the front side of the managing machine, a stopper shaft is fit in a hole and the back speed restriction piece 28b is moved to the movable range of the tilting rod 24 to inhibit the shift of the tilting rod 24 to the high-speed forwarding position and restrict the traveling speed. When the rotary is rotated by tilting a clutch lever forward, the back restriction piece 28a is moved to the movable range of the tilting rod 24 to inhibit the shift of the speed change lever 21 to the retreating position. When the clutch lever is pulled toward the worker to stop the rotary, the speed change lever 21 can be shifted to the backing position.