To facilitate a flexible change in the shape of a back inner plate and also to enhance the fitness to the human body, in a chair with a lumbar support function that the back inner plate is pushed out according to the physical structure of a person being seated.

The back inner plate 21 is provided with a side slit group 45 comprising a large number of slits 47 and 48. By the existence of the side slit group 45, the back inner plate 21 is changed in the shape easily with a curvature. While the back inner plate 21 is supported by right and left rollers, the portions of the plate with which the right and left rollers come into contact form a discontinuous part 49, and therefore a phenomenon of the rollers being caught in the slits 47 and 48 can be prevented.