To provide a non-formalin fiber filter having a countermeasure taken against formalin generation obtained by an optimum design of a weight per unit area and the diameter of a fiber of the filter.

In the fiber filter, which is mounted on a microwave-oven hood or a ventilation fan, the basis weight M and the diameter D of fiber are determined from the relationship between functional characteristics factors of the fiber filter composed of a collection factor C of oil vapor and a pressure drop P through the fiber filter in fan operation and structural characteristics factors composed of the basis weight M and the diameter D of fiber, which are determined only by the amount of the fiber material to be used, so as to meet a preset collection factor C and pressure drop P. Because the non-formalin fiber filter does not have a binding agent or hardsurfacing agent containing a urea-based resin and contains non-denatured guanidine phosphate, it generates no formalin components and is thus sanitarily and environmentally safe.