A chair backrest structure which fits well and looks good. The backrest structure has a back frame (20) extending upward from the end side of a seat part supported by a leg and a flexible back plate (21) which is provided in the back frame (20) and has a large number of diamond-shaped holes (24) in the form of a net. The back plate (21) is generated in the form of a net in which oblique yarns or belts are knitted, so that when a user's weight acts on the back plate (21), the enlogation amount of the back plate (21) is larger than that of a back plate having vertically long holes of the same width. This constitution makes the backrest fit well and look good.