The present invention addresses the problem of providing a filter medium that has a high dust capturing efficiency and a low pressure loss. The present invention is a filter medium that contains electrostatically-spun ultrathin fibers and that is characterized in that the ultrathin fibers contain multiple ester bonds in a molecule thereof. It is preferable that the average fiber diameter of the ultrathin fibers fall within a range of 10-5000 nm, and that the ultrathin fibers are formed of polylactic acid, polyethylene terephthalate, polymethyl methacrylate or polycarbonate. In addition, provided is a method for manufacturing a filter-medium fibrous structure, the method including a step for preparing a spinning solution in which a resin that contains multiple ester bonds in a molecule thereof is dispersed or dissolved in a solvent, and a step for obtaining a fibrous structure containing the ultrathin fibers by performing electrostatic spinning by using said spinning solution.