To operate a change gear operating member with an operating force smaller than conventional one and uniformed within the operating range of this change gear operating member.

The rotation of a motor is transmitted through a hydraulic driving change gear 14 to a traveling device and an output change input shaft 20 of the hydraulic driving change gear 14 is rotationally displaced while being linked with a change gear operating member 5. Concerning such a mobile farm machine, a cam plate 30 forming a guide plane (d) in a specified form is fixed to the input shaft 20, a driven element 32 capable of displacement on a specified track is brought into press-contact with the guide plane (d) by the elastic power of an elastic member 33 and in this case, the guide plane (d) is formed so as to uniform the required position holding power of the input shaft 20 within the displacement range of this input shaft 20.