A screw feeder includes: a case that includes a material putting-in portion through which a fluff material is put in and a material putting-out portion through which the fluff material is put out below the material putting-in portion; and a screw that includes a rotational shaft and a blade portion, the rotational shaft being provided rotatably inside the case, the blade portion being spirally mounted on the rotational shaft; wherein the screw is rotated with repeated alternation of a slope-down direction in which the blade portion slopes downward spirally in a vertical direction and a slope-up direction in which the blade portion slopes upward spirally in the vertical direction.