A hinge assembly adapted for a housing having a main body and a sub-body rotatably positioned on the main body, the hinge assembly comprising: a first hinge portion adapted to rotatably support both lower sides of the sub-body comprising first and second lower sides of the sub-body, wherein the first and second lower sides are located on opposite sides of the sub-body, and each of the first and second lower sides being on a face of the sub-body facing the main body; and a second hinge portion adapted to rotatably support a rear surface of the sub-body with a surface contact and to maintain a rotated angle of the sub-body wherein the second hinge portion comprises: a curved portion protruding from a bottom surface of the sub-body and having a gear groove arranged on the surface thereof; a window arranged on the sub-body rotational space of the main body and passing through the curved portion; and a pinion rotatably positioned at a predetermined position, the pinion arranged to be rotated together with the sub-body upon the sub-body being rotated in engagement with the gear groove; wherein the hinge assembly further comprises a boss protruding from the inner surface of the main body from a rear side of the window and a bracket, wherein the pinion is arranged to be rotatable by the boss protruding from the inner surface of the main body from a rear side of the window and by the bracket supported by the boss at a predetermined height and rotatably supporting a shaft of the pinion. a first hinge portion adapted to rotatably support both lower sides of the sub-body comprising first and second lower sides of the sub-body, wherein the first and second lower sides are located on opposite sides of the sub-body, and each of the first and second lower sides being on a face of the sub-body facing the main body; and a second hinge portion adapted to rotatably support a rear surface of the sub-body with a surface contact and to maintain a rotated angle of the sub-body wherein the second hinge portion comprises: a curved portion protruding from a bottom surface of the sub-body and having a gear groove arranged on the surface thereof; a window arranged on the sub-body rotational space of the main body and passing through the curved portion; and a pinion rotatably positioned at a predetermined position, the pinion arranged to be rotated together with the sub-body upon the sub-body being rotated in engagement with the gear groove; wherein the hinge assembly further comprises a boss protruding from the inner surface of the main body from a rear side of the window and a bracket, wherein the pinion is arranged to be rotatable by the boss protruding from the inner surface of the main body from a rear side of the window and by the bracket supported by the boss at a predetermined height and rotatably supporting a shaft of the pinion. a curved portion protruding from a bottom surface of the sub-body and having a gear groove arranged on the surface thereof; a window arranged on the sub-body rotational space of the main body and passing through the curved portion; and a pinion rotatably positioned at a predetermined position, the pinion arranged to be rotated together with the sub-body upon the sub-body being rotated in engagement with the gear groove; wherein the hinge assembly further comprises a boss protruding from the inner surface of the main body from a rear side of the window and a bracket, wherein the pinion is arranged to be rotatable by the boss protruding from the inner surface of the main body from a rear side of the window and by the bracket supported by the boss at a predetermined height and rotatably supporting a shaft of the pinion.