To constitute a planar elastic body easily stretchably within the frame of a seat frame by setting the planar elastic body at suitable tension and maintaining the specified tension free of variation.

A supporting frame plate 5 for holding the planar elastic body 2 at its circumferential edge is resin molded integrally with the planar elastic body 2 and the outer periphery of the supporting frame plate 5 is provided with detaining projections 6 of an approximately arrowhead shape in cross section at each of plural specified intervals. The seat frame 1 is provided with engaging holes 1a for these detaining projections in its axial direction at each of the plural specified intervals. The supporting plate 5 is applied and fixed to the front surface side of the seat frame 1 by fitting the respective detaining projections 6 into the engaging holes 1a of the seat frame 1. As a result, the planar elastic body 2 is stretched and mounted within the frame of the seat frame 1 by means of the supporting frame plate 5.