A trimming apparatus trims by cutter blades running from one end of a sheet bundle to the other end thereof, includes a bed surface for the sheet bundle, cutter blades for cutting the sheet bundle supported on the bed surface, a drive device for running the cutting blades from one end of the sheet bundle to the other end thereof, a support member having a support face, and a shift device for moving the support member between a working position for the support member supporting cut sheet dust pieces and a retreating position not to hinder the cut sheet dust pieces from dropping. The shift device displaces the support face from the working position to a retreating position after the cutter blades move at a predetermined distance toward the other end of the sheet bundle from one end thereof.