A method for producing a medium includes forming a fiber assembly by discharging a raw material liquid of fibers from a nozzle to generate the fibers and depositing the fibers so as to surround a circumferential surface of a rotatable winder, and transferring the fiber assembly to a base member while rotating the rotatable winder. The circumferential surface of the rotatable winder has a plurality of belt-shaped projection portions extending in a direction along a rotation axis of the rotatable winder.