To obtain a polyester-based fiber-structured material, modified by graft polymerizing a hydrophilic vinyl monomer, 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 heat-treating either one of a fiber-structured material consisting of a polyester-based composite fiber in which a polybutyleneterephthalate or a polypropyleneterephthalate occupies ≥20% and <80% fiber cross section, or a fiber-structured material consisting of a polyester mixed fiber containing ≥20% and <80% fiber consisting of the polybutyleneterephthalate or polypropyleneterephthalate, then imparting an aqueous solution containing a hydrophilic vinyl monomer and a polymerization initiator thereto, and heat- treating them.