To meet various needs as a chair, a chair used for a dialysis therapy, an examination table, or a stretcher by only one chair.
A supporting shaft 10 is placed as a protrusion on a side of a back frame 9 found on a seatback 4, and an armrest 11 is installed on the supporting shaft 10 in a manner that the armrest 11 can rotate around the supporting shaft 10. The armrest 11 can be a blow-molded body made of polypropylene, and is supported by a receiving part 12 and the supporting shaft 10 through a basic shaft 13. The basic shaft 13 has a catching means 25 which retains the armrest 11 at least in a horizontal state and the width of the armrest 11 in a vertical state. A stopper 26 is placed like a protrusion on a basic part 20 which keeps the basic shaft 13, and the basic shaft 13 would not rotate more than 180 degrees as the basic shaft 13 makes a contact with a receiving part 12 at a position where the basic shaft 13 is rotated 180 degrees through the armrest 11.