Precast concrete magnet is a new magnetic assembly which is used to fix the framework in precast industry. The assembly consists of some strong neodymium magnets and steel plates which form to certain designed magnetic circuit. This magnetic circuit provides very much stronger adhesive force to any ferrous workpiece. But the magnetic power is shielded and controlled by the thick metal box. We develop the push ON/OFF button to open or close the magnetic power outside of the metal box.