A folding portable device comprising a first case and a second case foldably connected to each other by a hinge portion, the first case and the second case being rotatable relative to each other between a closed state and an open state, the hinge portion comprising: a support body comprising a displacement shaft rotation portion, defining a displacement axis, that is rotatable around said displacement axis when the first case and the second case are rotated relative to one another around said displacement axis; an opening-closing support body defining an opening-closing axis that is orthogonal to the displacement axis and comprising an opening-closing shaft fixing portion and an opening-closing shaft rotation portion, wherein the opening-closing shaft fixing portion is fixed to the displacement shaft rotation portion, and wherein the opening-closing shaft fixing portion and the opening-closing shaft rotation portion are rotatable relative to each other around said opening-closing axis; a closed state engaging recess formed on the opening-closing shaft rotation portion; an open state engaging recess formed on the opening-closing shaft rotation portion; an opening-closing shaft spring sleeved on said opening-closing shaft fixing portion, wherein one end of the opening-closing shaft spring is connected to the opening-closing shaft fixing portion and the other end of the opening-closing shaft spring is connected to the opening-closing shaft rotation portion and further wherein the opening-closing shaft rotation portion is fixed to the first case; and an engaging projection portion formed on the first case, wherein, when the folding portable device is in the closed state the engaging projection portion is engaged with the closed state engaging recess and the opening-closing shaft spring is in a twisted state to provide a restoring force for rotating the opening-closing shaft rotation portion relative to the opening-closing shaft fixing portion and put the foldable portable device in the open state; and when the first case and the second case are rotated relative to one another around the displacement axis by a predetermined angle, the engaging projection portion disengages from the closed state engaging recess, the opening-closing shaft spring is released from the twisted state and the opening-closing shaft rotation portion is rotated around the opening-closing axis relative to the opening-closing shaft fixing portion by said restoring force thereby rotating the first case and the second case relative to one another so that the foldable portable device is in the open state and the engaging projection portion is engaged with the open state engaging recess. a support body comprising a displacement shaft rotation portion, defining a displacement axis, that is rotatable around said displacement axis when the first case and the second case are rotated relative to one another around said displacement axis; an opening-closing support body defining an opening-closing axis that is orthogonal to the displacement axis and comprising an opening-closing shaft fixing portion and an opening-closing shaft rotation portion, wherein the opening-closing shaft fixing portion is fixed to the displacement shaft rotation portion, and wherein the opening-closing shaft fixing portion and the opening-closing shaft rotation portion are rotatable relative to each other around said opening-closing axis; a closed state engaging recess formed on the opening-closing shaft rotation portion; an open state engaging recess formed on the opening-closing shaft rotation portion; an opening-closing shaft spring sleeved on said opening-closing shaft fixing portion, wherein one end of the opening-closing shaft spring is connected to the opening-closing shaft fixing portion and the other end of the opening-closing shaft spring is connected to the opening-closing shaft rotation portion and further wherein the opening-closing shaft rotation portion is fixed to the first case; and an engaging projection portion formed on the first case, wherein, when the folding portable device is in the closed state the engaging projection portion is engaged with the closed state engaging recess and the opening-closing shaft spring is in a twisted state to provide a restoring force for rotating the opening-closing shaft rotation portion relative to the opening-closing shaft fixing portion and put the foldable portable device in the open state; and when the first case and the second case are rotated relative to one another around the displacement axis by a predetermined angle, the engaging projection portion disengages from the closed state engaging recess, the opening-closing shaft spring is released from the twisted state and the opening-closing shaft rotation portion is rotated around the opening-closing axis relative to the opening-closing shaft fixing portion by said restoring force thereby rotating the first case and the second case relative to one another so that the foldable portable device is in the open state and the engaging projection portion is engaged with the open state engaging recess.