An armrest device for a chair includes a lower support rod that rises at a side of a chair seat and has a tubular upper end portion; and an upper support cylinder that has an armrest provided in an upper end thereof, is externally fitted to the lower support rod so as to be slidable in an up-down direction, and has a plurality of engaging portions therein. The armrest is made height-adjustable with respect to the lower support rod by disposing the locking member and a biasing member at an upper end portion of the lower support rod and providing the rotating rod inside the lower support rod.