A sheet feeding device includes: a sheet feeding tray attached to a casing in a manner slidable into and out from the casing and including a stacking plate on which sheets are stacked; a conveying unit configured to convey the sheets in a direction orthogonal to both a sliding direction of the sheet feeding tray and a stacking direction of the sheets; an optional extension unit configured to be removably attached to an upstream side of the stacking plate in a conveying direction, and including an extension plate configured to extend the stacking plate; and a first locking unit configured to be capable of being held in a first lock position where the sheet feeding tray cannot be slide when the optional extension unit is attached while, when the optional extension unit is removed, held in a first unlock position where the sheet feeding tray can be slid.