Abstract: A chair has a seat 1 and a backrest 2 and the backrest 2 is allowed to be tilted backward by elastic deformation of right and left back frame devices 4. The right and left back frame devices 4 each are composed of two unit frames 43 and 44. The back frame device 4 has a main portion 4a which are disposed outside the right and left sides of the seat 1, upper inward portions 43a and 44a which are connected to an upper rear end of the main portion 4a, lower inward portions 43b and 44b which go under the seat 1 from a lower end portion of the main portion 4a, and forward end portions 43c and 44c which are integrally connected to the lower inward portions 43b and 44b. The forward end portions 43c and 44c are fixed to the base 7.

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 chair has a seat 1 and a backrest 2 and the backrest 2 is allowed to be tilted backward by elastic deformation of right and left back frame devices 4. The right and left back frame devices 4 each are composed of two unit frames 43 and 44. The back frame device 4 has a main portion 4a which are disposed outside the right and left sides of the seat 1, upper inward portions 43a and 44a which are connected to an upper rear end of the main portion 4a, lower inward portions 43b and 44b which go under the seat 1 from a lower end portion of the main portion 4a, and forward end portions 43c and 44c which are integrally connected to the lower inward portions 43b and 44b. The forward end portions 43c and 44c are fixed to the base 7.

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 chair is provided with a base 6 fixed at an upper end of a leg pillar 5, a back support unit 13 attached to the base 6 so as to tilt backward, a backrest 3 attached to the back support unit 13, and a spring unit 14 supporting the backward tilt of the back support unit 13. The spring unit 14 is to expand and contract in a back and forth direction when viewed from above and arranged further behind than a tilt supporting point 16a of the back support unit 13. The back support unit 13, the spring unit 14, and the base 6 constitute a kind of link mechanism and the spring unit 14 will expand and contract mainly at the front end, while tilting backward.