To reduce fuel consumption of a tractor, and save cost on a truck.

This steering device comprises a king pin to simultaneously change directions of wheel increasing shafts (for four single tires in total) right and left to increase the maximum loading capacity by rotating a hydraulic oil pump to form pressure to a control valve, hub bearings, tie rods for connecting right and left small tires and large tires to each other, a load- carrying platform for a full trailer, and a hydraulic cylinder function to move the angle of the king pin in moving forward and backward. Two small tires can be increased on the inner side of the large tires where a frame is installed for increasing two small tires. A king pin angle adjusting cylinder operates in vertical directions in accordance with road surface conditions. Leaf springs are provided as shock absorbing devices. Since four single tires can be added in total, height of the load-carrying platform can be lowered.