To provide a detection method and device of a traction angle capable of omitting a wiring work of a tractor side angle sensor in attachment/detachment of the tractor and a trailer in a system in which the tractor tows the trailer such as a working machine through a swing draw bar and a hitch.

A link bar is arranged above the hitch for towing the trailer and rotation shafts at a tractor side end and a trailer side end of the link bar are subjected to bearing on the tractor and the trailer such that their axes are coincident with axes of rotation shafts at a tractor side end and a trailer side end of the hitch respectively. Angle sensors on the tractor side and the trailer side are provided on the rotation shafts at the tractor side end and the trailer side end of the link bar respectively. The traction angle of the tractor and the trailer required for controlling of a wheel steering device is detected based on the angle sensor.