The present invention relates to an anti-theft device which is a transmission line mounted on an object to be protected against theft, such as a door or a window, wherein a safety system is connected to the elongated metal to supply power to the anti-theft circuit of the elongated metal and to detect and judge the signal of the anti-theft circuit of the elongated metal. When the window or the door is broken, the anti-theft device will be easily torn at the respective apertures. At this moment, the safety system cannot receive the signal from the anti-theft device, so it will identify the intrusion of theft and notify the security or guard. Thereby providing an anti-theft function.