To obtain a modified polyester-based fiber-structured material, suppressing the reduction of its strength with time, and capable of fitting to the practical use, and to provide a method for producing the same.

This modified polyester-based fiber-structured material is obtained by graft polymerizing a hydrophilic vinyl monomer with a fiber- structured material consisting of at least 80 wt.% polypropyleneterephthalate. The method for producing such modified polyester-based fiber-structured material, is to impart an aqueous solution containing the hydrophilic vinyl monomer and a polymerization initiator to the fiber-structured material containing at least 80 wt.% polypropylene terephthalate and heat-treat them.