PURPOSE: To prevent the crops from being trodden by steering and controlling wheels of a working machine drawn through a hitch mechanism by a tractor so that the paths of the wheels of the working machine do not enter inside the paths of rear wheels of the tractor based on a hitch angle and steering angles of the front wheels of the tractor.

CONSTITUTION: A front wheel steering angle detector 23 is provided on one of front wheels, and a working machine wheel steering angle detector 25 is provided on a wheel 16 of a working machine drawn through a hitch mechanism by the tractor, a hitch angle detector 24 being provided on a hitch point of the hitch mechanism. Outputs of these detectors 23-25 are inputted to CPU26, and then a control to steer the wheels 16 of the working machine so that the working machine wheels 16 do not enter inside the paths of the rear wheels of the tractor is calculated referring to the front-rear wheels distance of the tractor, the distance from rear wheel axle of the tractor to a hitch point, and the distance from the hitch point to the working machine axle. Based on the difference between the control angle and the working machine wheel steering angle, a steering hydraulic cylinder 28 is driven and controlled through a steering control means 27.