PURPOSE: To reduce the facilities cost by providing a back door opening hanger on the hanger arm of a hanger conveyor for carrying the side member of a car body, and enabling the concurrent transport of the side member and the back door opening to a side member assembly machine.

CONSTITUTION: In assembling a body, an underbody A is carried to an assembly station 2 via a transfer 12. On the other hand, a common hanger conveyor 4 carries a side member B and a back door opening C to a side member assembly machine 3. ln this case, the carriage 19 of the assembly machine 3 is made to retreat and a turning frame 15 is turned to a horizontal position, thereby directing up receivers 23 and 24. In this state, a hanger arm 41 is lowered and each of members B and C is thereby delivered to the receivers 23 and 24. Thereafter, the turning frame 19 is turned to a vertical position, the carriage 15 is made to advance and the member B is aligned with the under body A. Also, the member C is aligned with the predetermined position between right and left side members B by means of a positioning device 25.