1. A mobile terminal comprising: a main device having a display and being shaped to define an area accessible via an opening; and a sub-device configured to be removably coupled to the main device via the opening, wherein the sub-device comprises a pen-type object, wherein the area is sized to receive an elongated portion of the pen-type object to permit coupling of the sub-device with the main device; a detection unit configured to detect decoupling of the sub-device from the main device; a controller configured to: cause the mobile terminal to transition to an unlocked state in response to the detection unit detecting the decoupling of the sub-device from the main device; and cause the display to display an image in response to the detecting of the decoupling of the sub-device from the main device.