Abstract: A method of mounting parts to both sides of a main body characterized by moving a carriage at a predetermined speed along a specified path of transport with the main body placed on the carriage; moving a pair of movable tables in synchronism with the carriage in the same direction as the movement thereof, the movable tables being disposed at opposite sides of the transport path and each carrying at least one robot and an assembling device; causing the movable tables to support the main body during the synchronized movement; setting a part held by the robot in a specified position on the main body; attaching the part to the main body by the assembling device; and thereafter delivering the main body from the movable tables onto the carriage. Since the carriage is moved during the assembly of parts, this method assures the assembly line of a high-speed operation.