A portable electronic device includes first and second device units and at least one hinge assembly configured to pivotably connect the second device unit to the first device unit. The hinge assembly includes a first fixing unit configured to be fixed to the first device unit, a second fixing unit configured to be fixed to the second device unit, a plurality of shaft members configured to be arranged in parallel with one another and to include first and second outer shaft members respectively mounted on the first and second fixing units and a plurality of inner shaft members arranged between the first and second outer shaft members, and a gear unit mounted on the shaft members to connect the plurality of shaft members with one another through a gear mesh.