To provide a comfortable seat and chair, eliminating discomfort such as pulling of clothes and shifting of the head from a headrest caused in tilting a backrest of a seat or a chair to be reclined.

Frames 2, 3 of the backrest seat 1 have a double structure, and a sliding frame 3 is provided, whereby in synchronization with tilting of the seat, a pin 13 mounted on a geared disk 12 for adjustment of an angle pushes down a grooved arm 8, the other arm 9 is pulled to expand and contract the backrest. The lower part of the backrest seat and the back of the seat are provided with cutouts 4, 5 to thereby prevent overlapping in expansion and contraction at that time, so that at the time of expansion and contraction, sliding along the cutout surface is performed. Further, this device has a unique effect that the pin 6 on the disk is moved while slid along the groove of the arm, whereby when the slide frame becomes near vertical or horizontal, the amount of sliding is reduced.