To provide an armrest which has high flexibility and plasticity and improves in maneuverability.

In a chair where an upper supporting frame 6 is formed above a lower supporting frame 2 laid on a floor and a back frame 9 and a seat frame 16 are set rotation-free on the upper supporting frame 6, an armrest frame 82 is structured to be attached to a side or both sides of the seat frame 16 so that it can switch between an armrest mode, in which the armrest moves back and forth keeping approximate horizontal state according to rotation of the back frame 9 with an armrest linkage 56, and a rail mode, in which the armrest is fixed at a lateral predetermined position of seat frame 16 in spite of rotations of the back frame 9, and it is attached to the seat frame 16 so that it can easily move up and down and rotate around a vertical axis.