To accurately detect a relative roll angle of a trailer, and prevent roll over of a vehicle by providing a measuring means of the relative roll angle of the trailer of the tractor side.

This device comprises displacement sensors 13R and 13L laterally arranged in a pair on both sides of a central shaft on an rear upper surface of a chassis of a tractor, and respectively measuring a distance with a front under surface of a loading space of the trailer, a roll angle arithmetic operation part 14 for calculating the relative roll angle of the trailer to the tractor by taking in respective measuring results from these displacement sensors 13R and 13L, a roll over risk determining part 21 for determining risk of vehicle roll over from the size of the calculated relative roll angle or a degree of an angle change, a display device 23 and acoustic device 24 for giving a warning to a driver by using a risk determining result from this roll over risk determining part 21 to thereby contribute to directly and accurately detecting the relative roll angle of the trailer as well as to preventing roll over of the vehicle.