A robots (210A, 210B), which is fixed onto a base (20, 20B) having a rectangular shape, has a mechanical hand (211A, 211B). A plurality of robot tools, each of which is freely attached to the tip of the mechanical hand (211A, 211B) for pinching a part, and a plurality of trays (6) each containing assembling jigs to be used for assembling parts, are brought in a movable range (RA, RB) of the robot, and the assembling tools are set at a predetermined position within the movable range (RA, RB). Then, an assembling job for the parts is carried out by using the assembling tools thus set. With this arrangement, the exchanging processes of the assembling tools can be carried out without depending on the human hands, even when the type of the assembly product is changed. Consequently, it becomes possible to efficiently carry out the manufacturing process of a small number of many types of products.