Abstract: A system for assembling bottom parts for vehicles, in which the bottom parts are transported to a position below a vehicle body positioned and fixed and are assembled to the vehicle body. The system includes a transporter for circulating a pallet member holding thereon bottom parts and a plurality of threaded parts for fastening the bottom parts to the vehicle body, so that the pallet member is stopped below the vehicle body. A first feeder is disposed in the vicinity of the transporter to feed the threaded parts to the pallet member. A second feeder is also disposed in the vicinity of the transporter to feed the bottom parts to the pallet member. A lifter is disposed below the vehicle body to raise the lower the pallet member relative to the vehicle body. A turning machine is also disposed below the vehicle body for turning the threaded part to fasten the bottom part to the vehicle body when the pallet member is in its raised position.