To reduce the size of an armrest in an axial direction.

The armrest includes an arm frame R1 and a clutch (one-way clutch 1) which causes the arm frame R1 to be supported in a rotatable manner by a seatback. The clutch includes: an inner ring 2, which is fixed to one of the arm frame R1 and the seatback; an outer ring 3, which is disposed radially outside the inner ring 2 while being separated from the inner ring 2 and is fixed to the other of the arm frame R1 and the seatback; a movable piece (roller 71), which is located between the inner ring 2 and the outer ring 3 and housed in a pocket having a portion with radial width gradually reducing along one circumferential direction; a release member 4, which includes a release piece 41 disposed adjoining in one circumference direction side to the movable piece and which is disposed to be coaxially rotatable with both the inner ring 2 and the outer ring 3; and a release operation mechanism, which operates the release member 4 to release a lock state where the movable piece is held between inner walls of the pocket.