The present disclosure relates to a unit chair. The unit chair includes a link unit for adjusting a posture of a headrest, wherein the link unit includes: a first link member fixed to a backrest portion of the unit chair; a second link member having one end portion hinged to the first link member by a first hinge pin; and a first ratchet member formed on one surface of at least one of the first link member and the second link member and constraining motion of the second link member so that the second link member is rotatable only in one direction based on a central axis line of the first hinge pin, wherein the first ratchet member includes a first ratchet base portion including a plurality of saw teeth formed on one surface, arranged in a ring shape based on the central axis line of the first hinge pin, uplifted in a direction toward the central axis line, and inclined only in one direction, and having an opposite surface fixed to the first link member; and a first ratchet rotating portion including a plurality of saw teeth formed on one surface and at least partially engaged with at least some of the saw teeth of the first ratchet base portion and having an opposite surface fixed to the second link member.