This invention relates to an apparatus for automatically attaching and detaching such exterior parts as doors, a trunk, and a bonnet, etc. which are openably mounted to a vehicle body by way of such tightening device as pins, bolts, etc. on the assembling line of a vehicle. In a door attaching and detaching apparatus the position of pins for openably attaching a door on a vehicle body is detected by a pin position detection mechanism, and the position of the pin drawing out and inserting mechanism is compensated according to the position thereof. Then, the pins can be drawn out from and inserted in the vehicle body and the door by the corresponding pin drawing out and inserting mechanism. Furthermore, the position of the door holding mechanism is compensated according to the position of the pins detected by the pin position detection mechanism, thereby causing the door to be held by the corresponding door holding mechanism. In this case, the pin drawing out and inserting mechanism and the door holding mechaism are separately installed independently and are individually driven to actuate.