A sheet post-processing device, comprising: a processing tray that accepts and loads thereon a prescribed number of sheets; a discharge tray that is disposed on a downstream side of the processing tray in a sheet conveyance direction, and onto which sheets loaded on the processing tray are discharged; and a discharge roller pair that is composed of a discharge roller that comes in contact with an uppermost surface of sheets loaded on the processing tray and a discharge roll that comes in contact with a lowermost surface of the sheets loaded on the processing tray and that is rotatable only in such a direction as to discharge the sheets onto the discharge tray, wherein a sheet is discharged from the processing tray onto the discharge tray by forwardly rotating the discharge roller pair, and a sheet is drawn in on the processing tray by reversely rotating the discharge roller in a state where the discharge roll is stopped from rotating, characterised in that the sheet post-processing device further comprises a roller holder that is swingable and rotatably supports the discharge roller, and the roller holder swings so that the discharge roller is selectively disposed at a retracted position away from the discharge roll and a contact position where the discharge roller comes in contact with the discharge roll, wherein the sheet post-processing device further comprises a biasing member that biases the roller holder downward, wherein in drawing in a sheet on the processing tray, the discharge roller is disposed at the contact position by using only a self-weight of the roller holder, and in discharging a sheet onto the discharge tray, the discharge roller is disposed at the contact position by, besides using the self-weight of the roller holder, biasing the roller holder downward using the biasing member. a processing tray that accepts and loads thereon a prescribed number of sheets; a discharge tray that is disposed on a downstream side of the processing tray in a sheet conveyance direction, and onto which sheets loaded on the processing tray are discharged; and a discharge roller pair that is composed of a discharge roller that comes in contact with an uppermost surface of sheets loaded on the processing tray and a discharge roll that comes in contact with a lowermost surface of the sheets loaded on the processing tray and that is rotatable only in such a direction as to discharge the sheets onto the discharge tray, wherein a sheet is discharged from the processing tray onto the discharge tray by forwardly rotating the discharge roller pair, and a sheet is drawn in on the processing tray by reversely rotating the discharge roller in a state where the discharge roll is stopped from rotating, characterised in that the sheet post-processing device further comprises a roller holder that is swingable and rotatably supports the discharge roller, and the roller holder swings so that the discharge roller is selectively disposed at a retracted position away from the discharge roll and a contact position where the discharge roller comes in contact with the discharge roll, wherein the sheet post-processing device further comprises a biasing member that biases the roller holder downward, wherein in drawing in a sheet on the processing tray, the discharge roller is disposed at the contact position by using only a self-weight of the roller holder, and in discharging a sheet onto the discharge tray, the discharge roller is disposed at the contact position by, besides using the self-weight of the roller holder, biasing the roller holder downward using the biasing member.