PURPOSE: To improve steering performance and lower a load-carrying platform by constituting a king pin in the load-carrying platform of a trailer connected to the coupler part of a tractor, in such a way as to be rotatable according to the rotation of the tractor, and transmitting the rotational displacement mechanically to a rear axle.

CONSTITUTION: The front face lower part in the load-carrying platform 16 of a trailer 14 is protrusively provided with a king pin 18. The king pin 18 is fixed to a rotary plate 20 and a bevel gear 22 rotated on the horizontal plane. Accordingly, when the king pin 18 is engaged with the coupler part of a tractor 28, the rotary plate 20 and the bevel gear 22 are also rotated according to the rotation of the tractor 28, and the rotational displacement is transmitted to a bevel gear 24 rotated on the vertical plane. Since the rotational displace-ment transmitted to the bevel gear 24 is transmitted to a hydraulic control valve 26, oil pressure from a hydraulic drive source 30 is force-fed to a flow control valve 34 through hydraulic piping 32 and then force-fed to hydraulic cylinders 38 through hydraulic piping 36. As a result, wheels 46 are steered through a transmission means.