PURPOSE: To further reduce the size of the rotation displacement transmitting device of a tractor by a method wherein a king pin to transmit rotation of a tractor is arranged in a protruding state to the lower front part of a trailer and coupled to a gear, rotated togetherwith a king pin, through a timing belt and a gear to operate a hydraulic mechanism through a hydraulic control valve is provided.

CONSTITUTION: A gear 7 located above a downward protruding king pin 6 and rotated togetherwith the king pin 6 and a gear 8 coupled to the gear 7 through a timing belt 9 are disposed to the front F of a trailer 1. The king pin 6 and a rotary plate 24 are rotated according to rotation of a tractor 2 and the gear 8 is rotated by the gear 7 through the timing belt 9. According to a rotation amount thereof, an oil pressure control device 10 is operated and an oil pressure from a hydraulic drive source 23 arranged to the tractor 2 is fed to a hydraulic mechanism 5 through oil pressure pipings 22 and 21, a flow rate control valves 11, and an oil pressure piping 20. The oil pressure is fed with a pressure to hydraulic cylinders 12 and 13 to rotate the rear wheels 3 and 4 of a trailer.