A sheet post-processing device characterized by: a processing tray which receives a predetermined number of sheets to have the sheets loaded thereon; a post-processing mechanism (61, 62) which performs predetermined post-processing with respect to a sheet bundle loaded on the processing tray; a discharge roller pair which includes a drive roller and a driven roller which follows the drive roller to rotate, and which discharges the sheet bundle loaded on the processing tray; a bundle discharge member which is reciprocatable along a loading surface of the processing tray, and which discharges the sheet bundle loaded on the processing tray by pushing out the sheet bundle; a loading tray which is arranged on a downstream side of the processing tray with respect to a discharge direction of the sheet bundle, and on which the sheet bundle having been discharged by the discharge roller pair or by the bundle discharge member is loaded; a sheet holding member which is arranged below the discharge roller pair, and which is swingable between a holding position at which the sheet holding member holds an upstream part, in the discharge direction, of the sheet bundle loaded on the loading tray and a retraction position at which the sheet holding member releases holding of the sheet bundle; a tray lifting-lowering drive portion which lifts and lowers the loading tray; a discharge drive portion which drives the discharge roller pair and the bundle discharge member; a sheet holding drive portion which swings the sheet holding member to the holding position and to the retraction position; and a control portion which controls the tray lifting-lowering portion, the discharge drive portion, and the sheet holding drive portion, characterized in that in a case where a number A of sheets included in the sheet bundle loaded on the processing tray is smaller than a predetermined number A1, the control portion discharges the sheet bundle onto the loading tray by using the discharge roller pair, with the loading tray arranged at a reference position, and in a case where the number A of the sheets included in the sheet bundle is equal to or larger than the predetermined number A1, the control portion discharges the sheet bundle onto the loading tray by using the bundle discharge member, with the loading tray having been lifted above the reference position. a processing tray which receives a predetermined number of sheets to have the sheets loaded thereon; a post-processing mechanism (61, 62) which performs predetermined post-processing with respect to a sheet bundle loaded on the processing tray; a discharge roller pair which includes a drive roller and a driven roller which follows the drive roller to rotate, and which discharges the sheet bundle loaded on the processing tray; a bundle discharge member which is reciprocatable along a loading surface of the processing tray, and which discharges the sheet bundle loaded on the processing tray by pushing out the sheet bundle; a loading tray which is arranged on a downstream side of the processing tray with respect to a discharge direction of the sheet bundle, and on which the sheet bundle having been discharged by the discharge roller pair or by the bundle discharge member is loaded; a sheet holding member which is arranged below the discharge roller pair, and which is swingable between a holding position at which the sheet holding member holds an upstream part, in the discharge direction, of the sheet bundle loaded on the loading tray and a retraction position at which the sheet holding member releases holding of the sheet bundle; a tray lifting-lowering drive portion which lifts and lowers the loading tray; a discharge drive portion which drives the discharge roller pair and the bundle discharge member; a sheet holding drive portion which swings the sheet holding member to the holding position and to the retraction position; and a control portion which controls the tray lifting-lowering portion, the discharge drive portion, and the sheet holding drive portion, characterized in that in a case where a number A of sheets included in the sheet bundle loaded on the processing tray is smaller than a predetermined number A1, the control portion discharges the sheet bundle onto the loading tray by using the discharge roller pair, with the loading tray arranged at a reference position, and in a case where the number A of the sheets included in the sheet bundle is equal to or larger than the predetermined number A1, the control portion discharges the sheet bundle onto the loading tray by using the bundle discharge member, with the loading tray having been lifted above the reference position.