To switch an armrest in a horizontal posture extending long in the front and back and an inclined posture which is long in the right and left and inclines forward through one action.

An arm support 10 inclines forward with the angle of about 12 degrees with respect to a vertical line by the side view of a chair. The arm support 10 has a circular cross section, and an arm reception outer cylinder 11 with the circular cross section and an arm reception inner cylinder 12 are engaged with the support 10 so that they can freely rotate and can freely move upward/downward. The armrest 13 is fitted to the arm reception cylinders 11 and 12. Since the arm support 10 inclines forward, the armrest 13 inclines forward to a seat when the arm reception cylinders 11 and 12 are rotated by 90 degrees. Even if a seated person works by stretching the arms, the arms are prevented from slipping from the armrests and the arms can precisely be supported.