A line sychronizing apparatus includes a conveyer 1 for conveyance of a work, a first transport rail 2 mounted above and in parallel with the conveyer, a main assembly 10 slidably supported on the first transport rail by a plurality of rollers, a second transport rail 14 mounted in parallel with the conveyer, and an operating assembly P slidably supported by the second transport rail by a plurality of rollers. The apparatus prevents the heavy weight of the main body of the apparatus from being applied to the work. The apparatus performs each assembling operation in a synchronized condition while the work is being transported.