To detect a relative attitude angle with good accuracy and favorable responsiveness at low cost without applying special processing to a vehicle to be pulled in a relative attitude angle detection device for detecting a relative attitude angle of a pulling vehicle (tractor) and a vehicle to be pulled (trailer).

Displacement sensors 50-1, 50-2 detect relative displacement of a second reference plane of the vehicle 30 to be pulled in a vertical direction against a first reference plane 12 of the pulling vehicle 10 respectively and rotation angle sensors 51-1, 51-2 detect movement displacement in the second reference plane direction at the detection points of the displacement sensors 50-1, 50-2 respectively. A displacement sensor 50-3 is installed at a position by a predetermined distance in a longitudinal direction of the pulling vehicle 10 from the displacement sensors 50-1, 50-2 and detect relative displacement of the second reference plane in the vertical direction against the first reference plane 12 of the pulling vehicle 10. An operation part 70 operates the relative attitude angle between both vehicles based on the detection result of the displacement sensors 50-1-50-3 and rotation angle sensors 51-1, 51-2.