A portable electronic device having a sliding housing and adapted to slide in multiple directions so that it can be used as a key input device is provided. The portable electronic device includes a main housing, a sliding housing coupled to the main housing while being able to slide in upward, downward, leftward, and rightward directions on the main housing and at least one motion sensor for sensing a movement of the sliding housing in each direction, for creating a corresponding signal, and for applying the signal to a controller inside the main housing. The sliding housing provides a user input so that additional keys are not required and the overall size of the device can be reduced.