An armrest is fixed to a shaft (20) of a one-way clutch (1) that is provided on a seat frame, and is rotatable together with the shaft (20) relative to the seat frame. The armrest is configured to be switchable by the one-way clutch (1) between a locked state in which rotation of the armrest in a downward direction is restricted and an unlocked state in which rotation of the armrest in upward and downward directions is allowed. The armrest includes a main frame (100) constituting a framework of the armrest, and a reinforcement frame (200) having a thickness larger than that of the main frame (100), the reinforcement frame (200) and the main frame (100) lying one over another. The reinforcement frame (200) is fixed to the shaft (20).