To provide a seat for a chair which has a seat cushion repeatedly attached and detached, achieves a long life of the seat cushion, has a seat plate easy to wipe, and which facilitates positioning the seat cushion to the seat plate, and to provide a folding chair using the same.

The folding chair 1 has the seat plate 8 formed with through-hole groups 11 having three through-holes 10 at the four corners. The seat cushion 9 has a seat core plate 15 formed with fitting projection groups 17 fitted into the through-hole groups 11. The seat plate 8 is provided with a cylindrical bodies 18 on the lower surface to enclose the fitting projection groups fitted into the through-hole groups. The fitting projection group 17 has three fitting projections 16, a tapping screw 20 is threaded into one of the fitting projections 16, and the seat cushion 9 is fixed to the seat plate 8. When the screw is loosened, another fitting projection 16 of the fitting projection group 17 may be used.