To solve a problem on a conventional complex and expensive mechanical trailer connecting angle detecting device formed by mounting a turn table and a lever to a king pin for detecting a connecting angle θ, with respect to a trailer 2 connected to a tractor 1 by means of a rotating connecting part 3, to be bent to the tractor 1.

A scan radar 4 is mounted on a roof of the tractor 1 to be horizontally scanned with respect to a trailer front panel 2F. As the strong laser reflection beam is returned from a central reflector 5FC mounted on the trailer front panel 2F, the direction of the existence of the central reflector 5FC (angle δFC) can be detected by the scan radar 4. As the following relationship is satisfied between the detected angle and the connecting angle θ, the connecting angle θ can be determined on the basis of this relationship: δFC= arctan{a.sinθ/(d-a.cosθ)} ((a) is distance between a rotating connecting part 3 and the central reflector 5FC).