Abstract: A fiber structure contains polyester fibers 1 and a hydrophilic polyester resin treatment agent, wherein part 2a of a molecule 2 of the hydrophilic polyester resin treatment agent is absorbed in at least part of the polyester fibers 1, and the remainder 2b of the molecule extends along the surface of the polyester fibers 1 to hydrophilize the surface. The fiber structure has the properties below: (1) antifouling properties of grade 4 or higher according to the gray scale assessment determined by applying 200 g of muddy water, which is a mixture of normal staining soil and sand and distilled water at a ratio of 1:1, to the fiber structure, followed by standing for 24 hours, washing, and drying; (2) water absorbability of 10 seconds or less according to the falling-drop method; and (3) diffusivity of 55 minutes or less according to the diffusible residual water content test (dropping 0.6 mL of water).