PURPOSE: To improve adaptability of a tractor for change of a soil condition and/or existence of an obstacle, by constituting the tractor in such a manner that tractive resistance, when increased to at least a certain value, is detected to actute a speed reducer and automatically decrease a running speed of the tractor.

CONSTITUTION: In case of connecting a plow and the like to a tractor side 1 through an upper link 2 and lower link (not shown in the drawing), the lower end part of a bracket 4 is pivotally mounted to a tractor body 6 by a pivot 5, and the front end of the upper link 2 is pivotally mounted to an intermediate part of the bracket 4, then the upper end of said bracket 4 is connected to a rear end member 8a of a draft rod 8 by a shaft 7. A pressing element 8b, appearing to a switch 25 provided to the body 6, is provided to said rear end member 8a, and an applied load to the plow increases to at least a prescribed value, if the draft rod 8 is advanced against a spring 9, the switch 25 is turned ON. At ON time of said switch 25, a solenoid valve is opened, and a speed reducer (both valve and reducer not shown in the drawing) is operated to perform speed down running of the tractor.