A sheet trimming apparatus including: a trimming section provided with a trimming cutter blade for trimming a sheet bundle; a paper scrap container for storing paper scraps separated from the sheet bundle by the trimming cutter blade; and a paper scrap guide member, which is provided to create a falling space where the paper scraps are made to fall from the trimming section to the paper scrap container, wherein the paper scrap guide member assumes a first state for expanding a gap in the falling space, and a second state for creating an inclined surface for guiding the paper scraps to fall.