PURPOSE: To provide a control device for a steering device by which nonconformity such as a temperature rise in a motor, an oil temperature rise and large electric power consumption can be eliminated by generating oil pressure by maximum output at steering time of a trailer, and normally generating low oil pressure.

CONSTITUTION: In a steering device for a trailer to transmit the displacement to left and right rear wheels by a link mechanism through a steering panel of a wheel shaft by converting rotational displacement of a tractor into extensional and contractional displacement of a hydraulic cylinder 18 through oil pressure piping, the device has a control valve 40 having a characteristic to generate differential pressure in output by controlling the displacement of the cylinder at steering time of a trailer. Two relief valves 54 and 58 are connected in parallel to each other in oil pressure piping to connect the control valve and a hydraulic pump 48 to each other, and a solenoid valve 60 is connected in series to the relief valve whose designated pressure is low, and an electronic control device is provided to send a signal to an electric motor 46 and the solenoid valve according to differential pressure by detecting the differential pressure at output time of the control valve, and is constituted so that oil pressure is generated by maximum output at steering time of the trailer and low oil pressure is normally generated.