To allow a user to pass between both sidewalls from a rear backrest portion and sit on a chair while relying on armrest portions, to assemble the chair including the backrest through simple procedures, to immediately provide chair functions although the chair is normally in a folded state, and to allow easy passage by tightening both sidewalls by a method which does not necessitate steps.

A backrest portion 6 is designed to rotate downwards and a seat portion 4 is designed to rotate upwards in a manner such that they are rotated in conjunctions with each other, and an elastic force is normally utilized for folding them to a side face. To assemble the chair, a pushing force overwhelming the elastic force is applied and the portions are held by a locking device, with both side walls interconnected via a steel plate of a bottom face to maintain perpendicularity, and with casters provided for permitting movement. Armrest portions 2 on sidewalls are extended and restored to their proper positions and can be optionally placed on a table when necessary.