There is little fluctuation in amount of short fibers among individual products of fiber substrate. No damage is caused to a mold for the fiber substrate. Continuous production of the fiber substrates is possible. A method of manufacturing a fiber substrate includes the steps of: preparing slurry by dispersing short fibers in a dispersion medium; pouring the slurry into a cylindrical mold from above the cylindrical mold, the slurry being directed to a slurry diffusion member disposed in the center of the cylindrical mold and having an upward pointing conical or pyramidal shape; pouring the dispersion medium or water onto the slurry diffusion member from above the slurry diffusion member to cause the short fibers adhering to the slurry diffusion member to fall down, after the step of pouring the slurry; and discharging the dispersion medium from the cylindrical mold to accumulate the short fibers in the cylindrical mold to obtain a fiber aggregate.