To separate only an uppermost sheet of a stack with reliability regardless of the weight of a medium.

Coating 30 of a parallel rib extended from a base face of a dam is formed of a metal body and has a low friction coefficient to a sheet medium, and its outer surface has a longitudinal slot. An insert formed of polyurethane with a high friction coefficient and supported in a body is preloaded, and a protrusion 36 is protruded through the slot in the body in order to be engaged with each advancing sheet. In case of the rigid sheet, the protrusion is pressed into the body, and the low friction coefficient face of the body with a larger area than the insert with respect to sheet engagement is engaged with the advancing sheet. In case of the flexible sheet, the protrusion of the insert is engaged with the advancing sheet to buckle the sheet or make it wavy, and the sheet is engaged with the low friction face of the body and separated from a following sheet in a stack.