To continuously manufacture vehicles of different forms such as a vehicle provided with a four-wheel type traveling device and a vehicle provided with an endless track traveling device in mixed manufacture by the same assembling line.

A machine side supporting part for supporting the vehicle in floated states of the traveling devices is attachably/detachably provided on a truck, and left and right traveling device side supporting parts for supporting left and right traveling devices in mounted states are provided on the truck so that a space in a right/left direction may be changeable. While moving the truck along an assembly line, the supply of parts is received from a parts supply conveyor to assemble the vehicle. In a state in which the assembling is substantially completed and the moving of the truck is stopped, the traveling devices of the vehicle are grounded and separated from the truck by lowering the truck from a floor surface. After lowering the truck so that the left and right traveling device side supporting parts may have substantially the same height as that of the floor surface, the vehicles are traveled and are separated from the truck.