To fix a steering gear mechanically so as to make a trailer travelable as a normal trailer at the time of the failure of a battery area and a controller by providing a hydraulic unit for actuating a fixing hydraulic cylinder, separately from a hydraulic unit for actuating a steering hydraulic cylinder.

A steering gear for steering the rear wheels of a trailer is so constituted that the rotation of a steering plate is stopped by the expansion of a fixing hydraulic cylinder 34 in order to make the steering plate fixable even during travel to fix the steering gear. A steering hydraulic cylinder 18 connects a control valve 40 to a hydraulic pump 48 rotated by the operation of an electric motor 46, through pipelines 50, 52. In this case, a hydraulic circuit formed of an electric motor 80 for actuating the fixing hydraulic cylinder 34, a hydraulic pump 82 actuated by this electric motor 80, a manual pump 78, a relief valve 84 and an oil tank 84 is independently provided to make the fixing hydraulic cylinder 34 operable by this independent hydraulic circuit during the failure of a controller or the like.