To continuously perform mixed model production of a vehicle having a different form such as a vehicle provided with, for example, a four-wheel type traveling device and a vehicle provided with an crawler belt type traveling device by the same assembling line.

A vehicle body side support part for supporting the vehicle in the state that the traveling device is floated up is detachably provided on the truck and left and right traveling device side support parts for supporting the vehicle in the state that the left and right traveling devices are placed are provided on the truck so as to be capable of changing left/right clearance. Further, the vehicle supported on the truck is assembled while moving the truck along the assembling line and the traveling device of the vehicle supported on the machine body side support part is ground-contacted by descending the truck from a floor surface to separate the vehicle from the truck in the state that the assembling is approximately completed and movement of the truck is stopped. The vehicle is removed from the truck by traveling the vehicle after the truck is descended such that left and the right traveling device side support parts become the approximately same height as the floor surface.