PURPOSE: To make it possible to form the seat and back rest of a simple prefabricated seat consisting of side plate connecting beams, seat back rest between a pair of side plates to an arbitrary shape.

CONSTITUTION: Cords 15 are respectively spanned in a parallel state between the through-holes 12 of the seat-corresponding parts of both side plates 11 consisting of perforated plates having the many through-holes 12 apart spaced intervals longitudinally and transversely and the through-holes 12 of the back rest corresponding to parts, by which the seat 16 and back rest 17 consisting of the parallel cord groups are formed. The parallel cord groups forming the seat 16 and the back rest 17 may be covered with covers having softness.