Image forming apparatus having sheet cassette having lateral guides

A sheet feed cassette used in an image forming apparatus for storing a stack of a plurality of sheets and for delivering each one of the sheets to a desired position in a sheet feeding direction, the cassette comprising:

• a sheet guide unit having a guide surface for guiding movement of the sheet in the sheet feeding direction, lateral side edges of the sheet being abuttable on the guide surface, the sheet guide unit having an upper end;
• a sub-guide unit vertically movably provided at the sheet guide unit so as to be protrudable to a position above the upper end of the sheet guide unit, the sub-guide unit having an auxiliary guide surface flush with the guide surface of the sheet guide unit for guiding movement, in the sheet feeding direction, of an upper portion of the sheet stack when the sub-guide unit protrudes upwardly from the sheet guide unit, the upper portion of the sheet stack being positioned above the upper end of the sheet guide unit.