Sheet feeding device and image forming apparatus

A sheet feeding device comprising:

• a casing;
• a sheet feeding tray attached to the casing in a manner that the sheet feeding tray can be slid into and out from the casing and including a stacking plate on which sheets are to be stacked;
• a conveying unit configured to convey the sheets stacked on the stacking plate 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 to allow a long sheet to be stacked;
• and
• a first locking unit configured to be capable of being held in a first lock position where the sheet feeding tray cannot be slid 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.