Abstract: A front portion of a seat 2 is connected to a base 6 by a front support link 7 and a first support shaft 8, and a rear portion of the seat 2 is connected to a rocking frame 11 by way of a rear support link 14. The first support shaft 8 is slidably fitted to the base 6. An upper portion of the backrest 3 is attached to an upper portion of a back frame 12. When a person is seated, the seat 2 is moved down and moved back, and a lumber support portion 17a of the backrest 3 is moved forwardly in cooperation with movement of the seat 2. Therefore, even when a person is shallowly seated on a chair, the waist portion of the person can precisely be supported. The cooperatively moving means includes a cooperatively moving link mechanism comprising a first through a fifth cooperatively moving link 43, 46, 48, 21 and 20. The third link 48 can be pivoted to return against an elasticity of a second returning spring 49. Therefore, a person is not hampered from being seated thereon deeply.

Abstract: A front portion of a seat 2 is connected to a base 6 by a front support link 7 and a first support shaft 8, and a rear portion of the seat 2 is connected to a rocking frame 11 by way of a rear support link 14. The first support shaft 8 is slidably fitted to the base 6. An upper portion of the backrest 3 is attached to an upper portion of a back frame 12. When a person is seated, the seat 2 is moved down and moved back, and a lumber support portion 17a of the backrest 3 is moved forwardly in cooperation with movement of the seat 2. Therefore, even when a person is shallowly seated on a chair, the waist portion of the person can precisely be supported. The cooperatively moving means includes a cooperatively moving link mechanism comprising a first through a fifth cooperatively moving link 43, 46, 48, 21 and 20. The third link 48 can be pivoted to return against an elasticity of a second returning spring 49. Therefore, a person is not hampered from being seated thereon deeply.