A chair comprising: a leg erected on a floor surface; a seat arranged above the leg ; a backrest (306; 406) being arranged behind the leg and including a pressure receiving surface (PS); and a support mechanism (SM) arranged below the seat to be interposed between the leg and the seat and configured to movably support the seat in a front-rear and left-right direction along a predetermined trajectory and to downwardly incline a tip side of the seat in a movement direction in accordance with a movement of the seat from a predetermined reference position (S), wherein the support mechanism (SM) including a return-force generation unit configured to generate, in accordance with an amount of movement, a return force in a direction in which the seat moved from the reference position (S) in the front-rear or left-right direction is returned to the reference position (S), and being configured to movably support the backrest (306; 406) in the left-right direction together with a movement of the seat in the left-right direction, wherein the backrest (306; 406) includes a main body unit (361; 461), and an amplification element (370, 362y; 462, 462y) attached to the main body unit (361; 461) and configured to further amplify a movement operation of the main body unit (361; 461) in the left-right direction among movement operations of the main body unit (361; 461), and the amplification element (370, 362y; 462, 462y) is configured to perform an amplification operation independent from a movement operation of the main body unit (361; 461) in the left-right direction, wherein the amplification element (370, 362y; 462, 462y) includes a first amplification element (370; 462) configured to amplify a movement operation of the main body unit (361; 461) in the left-right direction, and a second amplification element (362y; 462y) configured to further amplify an operation of the first amplification element (370; 462). a leg erected on a floor surface; a seat arranged above the leg ; a backrest (306; 406) being arranged behind the leg and including a pressure receiving surface (PS); and a support mechanism (SM) arranged below the seat to be interposed between the leg and the seat and configured to movably support the seat in a front-rear and left-right direction along a predetermined trajectory and to downwardly incline a tip side of the seat in a movement direction in accordance with a movement of the seat from a predetermined reference position (S), wherein the support mechanism (SM) including a return-force generation unit configured to generate, in accordance with an amount of movement, a return force in a direction in which the seat moved from the reference position (S) in the front-rear or left-right direction is returned to the reference position (S), and being configured to movably support the backrest (306; 406) in the left-right direction together with a movement of the seat in the left-right direction, wherein the backrest (306; 406) includes a main body unit (361; 461), and an amplification element (370, 362y; 462, 462y) attached to the main body unit (361; 461) and configured to further amplify a movement operation of the main body unit (361; 461) in the left-right direction among movement operations of the main body unit (361; 461), and the amplification element (370, 362y; 462, 462y) is configured to perform an amplification operation independent from a movement operation of the main body unit (361; 461) in the left-right direction, wherein the amplification element (370, 362y; 462, 462y) includes a first amplification element (370; 462) configured to amplify a movement operation of the main body unit (361; 461) in the left-right direction, and a second amplification element (362y; 462y) configured to further amplify an operation of the first amplification element (370; 462). <EOS>