A weighty object mounting system includes a mounting table provided in a mounting station established in an assembly line for supporting a weighty object placed thereon to be mounted on a transportable object transported into the mounting station, a lifting device for lifting the mounting table to move the weighty object thereon into a mounting position provided on the transportable object, a position adjusting device for shifting the position of the mounting table so as to compensate for misalignments arising between the transportable object and the weighty object, a mounting table releasing device operative to cause the position adjusting device to make the mounting table movable in a plane along the direction of transportation of the transportable object in the mounting station, and a positioning device attached to the mounting table and having a tapered portion provided for engaging with a receiving portion provided on the transportable object when the mounting table ascends. A sensor is also provided for detecting such a condition that the tapered portion of the positioning device comes into engagement with the receiving portion of the transportable object, and the mounting table releasing device is controlled by a controller to operative in response to a detection output from the sensor.