To prevent generation of a work error so as to improve working ratio of equipment by changing a working position and performing work again until a rerun frequency counting result reaches a set frequency if defective work is determined by means of a good/defective determining means.

A plurality of robots 1, 2 are arranged toward a conveyor 3, while working benches 4, 5 are arranged between the robots 1, 2 and the conveyor 3, and part supplying devices 8, 9 and the like are arranged in the robots 1, 2. Parts placed on a pallet 10 and the like conveyed on the conveyor 3 are picked up by means of the robots 1, 2 so as to be transferred to the working benches 4, 5, and the parts fed from the part supplying devices 8, 9 are assembled on the working benches 4, 5 and returned to the pallet 10 again. According to controllers 6, 7 for the robots 1, 2, a working position is changed by means of a working position changing means and work is carried out again until a counting result of a rerun frequency counting means reaches the predetermined frequency, if defective work is determined by means of a good/defect determining means.