To realize small-quantity, ready and low cost production of lengthy object capable of carrying semi-trailer automobiles.

In the semi-trailer automobile, a coupler 13 is disposed in a chassis frame 14 of a length L near a length size limit Ls with an overhung section 15 projecting rearward from a rear wheel 12 on the rear wheel 12 of a tractor 10 for a pole trailer. A loading platform frame 23 has a length M from its lower surface fixing king pin 26 detachably engaging with the coupler 13 to its rear end 25 near a length size limit Ms. The frame 23 is extended forward to a position apart from a cabin 17 to form an overhang part 24. The automobile has a steering interlocking device turning a trailer rear wheel 22 in conjunction with a tractor front wheel 11.