Reaction force mechanism for backrest of a chair has clear large space under the seat and uses a longer reaction force spring than the prior art. The mechanism includes a base member supported on a leg, a back support member coupled by a rotation shaft to the base member to recline and support the backrest, a seat support member to which the seat is mounted, a weight-dependent reaction force mechanism for moving the seat support member in a lifting direction, and a reaction force spring for applying a spring force for returning the back support member to an original position, in which the reaction force spring is disposed in a lateral orientation between the back support member and the seat support member.