An electronic device comprises an outer case, a wireless card reader and a signal processing device. The outer case comprises a door and a metal wall. The metal wall is disposed on a lateral side of the outer case and has an opening. The door is disposed on the metal wall and capable of covering the opening. The wireless card reader is inside the door and capable of transmitting wireless signals. The signal processing device has a case, a data storage device and a safety mechanism. The signal processing device is inside the outer case and is electrically connected to the wireless card reader. The signal processing device is capable of processing the data of wireless signals from the wireless card reader. When the case is dissembled, the safety mechanism destroys at least part of the data stored in the data storage device.