A packing box made of a paper-made corrugated board sheet, comprising: a bottom panel; end panels (13A, 13B) raised from first edges of the bottom panel; side panels (15A, 15B) raised from second edges of the bottom panel extending in a direction which intersects with the first edges, the side panels (15A, 15B) arranged adjacently to the end panels (13A, 13B); edge panels connected to side edges of the end panels (13A, 13B) via outer panel fold lines, and fixed to inner side surfaces of the side panels (15A, 15B) arranged adjacently to the edge panels; fold-back panels arranged on inner side surfaces of the edge panels; inner panels arranged on inner side surfaces of the end panels (13A, 13B); and reinforcing panels continuously formed with the fold-back panels via first fold lines, and continuously formed with the inner panels via second fold lines, wherein a third fold line is provided between the first and second fold lines (29, 30) of each reinforcing panel, the third fold line extending parallel to the first and second fold lines (29, 30), wherein each reinforcing panel is formed in an inwardly projecting manner between the fold-back panel and the inner panel to form a projecting top portion, wherein any of the first fold line and the second fold line positioned on a side closer to the outer panel fold line than the projecting top portion is arranged away from the outer panel fold line by a distance, characterized in that a stage collapsed portion where a panel thickness of the paper-made corrugated board sheet is reduced by compression is formed on each reinforcing panel with a predetermined width so as to include the third fold line. a bottom panel; end panels (13A, 13B) raised from first edges of the bottom panel; side panels (15A, 15B) raised from second edges of the bottom panel extending in a direction which intersects with the first edges, the side panels (15A, 15B) arranged adjacently to the end panels (13A, 13B); edge panels connected to side edges of the end panels (13A, 13B) via outer panel fold lines, and fixed to inner side surfaces of the side panels (15A, 15B) arranged adjacently to the edge panels; fold-back panels arranged on inner side surfaces of the edge panels; inner panels arranged on inner side surfaces of the end panels (13A, 13B); and reinforcing panels continuously formed with the fold-back panels via first fold lines, and continuously formed with the inner panels via second fold lines, wherein a third fold line is provided between the first and second fold lines (29, 30) of each reinforcing panel, the third fold line extending parallel to the first and second fold lines (29, 30), wherein each reinforcing panel is formed in an inwardly projecting manner between the fold-back panel and the inner panel to form a projecting top portion, wherein any of the first fold line and the second fold line positioned on a side closer to the outer panel fold line than the projecting top portion is arranged away from the outer panel fold line by a distance, characterized in that a stage collapsed portion where a panel thickness of the paper-made corrugated board sheet is reduced by compression is formed on each reinforcing panel with a predetermined width so as to include the third fold line.