A key input device for a portable communications apparatus having a main body housing and a sliding body housing mounted on the main body housing to be slid along first and second directions perpendicular to each other. The key input device includes a first key array arranged on the main body housing to be exposed or covered by the sliding motion of the sliding body housing along the first direction, a second key array arranged on the main body housing to be exposed or covered by the sliding motion of the sliding body housing along the second direction, and a third key array arranged on the main body housing to be exposed or covered by the sliding motion of the sliding body housing along either the first or the second direction, wherein the first, second and third key arrays are all exposed or covered when the sliding body housing is slid first along the first direction and then along the second direction, or first along the second direction and then along the first direction.