A fiber processing device includes a drum having a plurality of openings on an outer peripheral surface, sieving a raw material containing fibers, and configured to rotate about a rotation axis, a housing that supports the drum, a guide portion (convex portion) provided on an outer peripheral surface of the drum; and a restriction portion (bearing) that restricts movement of the drum in a direction of the rotation axis by contacting the convex portion. At least one of the restriction portion and the guide portion is configured to rotate about an axis (rotation axis) orthogonal to the rotation axis.