A sheet manufacturing apparatus includes a rotatable drum unit that includes an opening section having a plurality of openings on a surface thereof and a cylindrical section having no opening; a material supply unit that is provided to supply a material containing fibers to the drum unit by airflow; and a forming unit that forms a sheet by using the material passing through the openings, in which at least one of the drum unit and the material supply unit has a diffusion section in which airflow is diffused further on a downstream side than on an upstream side in a supply direction of the airflow.