A portable digital communication apparatus including a first housing extending along a longitudinal direction; a second housing which is shorter than the first housing and which is adapted to be opened away from or closed on the first housing through sliding and rotation motions; and a dual-axis hinge device connecting the first and second housings to allow them to be rotated about a first hinge axis, which is provided in the first housing, and a second hinge axis, which is provided in the second housing and which is parallel to the first hinge axis, respectively, the dual-axis hinge device being adapted to be rotated to allow the upper and lower surfaces thereof can be reversed according to the sliding motion of the second housing.