PURPOSE: To ensure easy control, make a drive mechanism compact and reduce facility cost by synchronously shifting three shuttle bars via a single drive mechanism without providing any connection member.

CONSTITUTION: In the conveying device of a body floor main assembly line having two shuttle bars 15 and one shuttle bar 17 laid in series, two idle bars 18 extended along the shuttle bar 17 are connected to the two shuttle bars 15. The two idle bars 18 and single shuttle bar 17 are provided with racks. The three racks are respectively engaged with pinions on a common drive shaft rotatably driven with a drive mechanism 19. The rotation of the common drive shaft causes the simultaneous advance and retreat of the three racks, while three pinions are being interlocked with one another. Consequently, the two shuttle bars 15 driven via the two idle bars 18 advance and retreat, and the single shuttle bar 17 also advances and retreats synchronously.