Abstract: The present invention relates to a safety device for an upper die arbor of a fastening machine having a driving sleeve, an upper mold mandrel, a copper sleeve, and a steel sleeve. The driving sleeve has a roller in each of four holes. A flange is formed on the circumference of upper mold mandrel. Due to the positions of the rollers with respect to the flange, and the difference between inside diameters of quadrangular slots formed in the copper sleeve and steel sleeve, when the upper mold mandrel moves downward and is blocked by an undesired object, the rollers pass over the flange and do not exert the normal forming force on the upper mold mandrel. Therefore, the upper mold mandrel does not exert the mechanical crushing force in order to prevent injury to the object.