To solve the problem that a conventional device for detecting the angle of coupling of a trailer is an expensive mechanical device of a complicated structure constituted by mounting a turn table and a lever to a king pin for detecting the angle θ of coupling in the case that the trailer coupled to a tractor 1 by a rotating connecting part 3 is bent into the state of the trailer 2-2.

Cameras 7 and 8 for placing the rear ends 5 and 6 of the trailer in the photographic field of view on each side are mounted to both sides of the tractor 1. The locational relationship when the trailer is bent is shown in Fig. By geometrically analyzing the shape of the diagram in Fig., it is found out that an angle δt facing the rear end 6 of the trailer and the angle θ of coupling have the following correlation. By obtaining δt from the image location of the rear end 6 of the trailer in a photograph screen and utilizing the correlation, it is possible to obtain the angle θ of coupling. δt= arctan[{A×sin(θ+)-h}/{d+A×cos(θ+)}]+δs.