A sheet post-processing apparatus comprising: a processing tray configured to load a sheet; a vertical alignment section that is fixedly attached to a rotational shaft, configured to align the sheet on the processing tray in a sheet conveying direction by rotating around the rotational shaft; a horizontal alignment section configured to align the sheet in a sheet width direction orthogonal to the sheet conveying direction, by sandwiching with a first alignment member and a second alignment member; conveying rollers (223a, 223b) configured to convey a sheet; a discharge tray; and a controller configured to control the horizontal alignment section to shift the sheet to a first alignment position a predetermined distance from the center of the processing tray in the sheet width direction, control the horizontal alignment section to align the sheet in the sheet width direction at the first alignment position, control the vertical alignment section to align the sheet in the conveying direction at the first alignment position, and control the horizontal alignment section to realign the sheet at the first alignment position; characterized in that the controller is further configured to determine whether a sheet conveyed to the processing tray is a last sheet, when the controller determines the sheet conveyed to the processing tray is not the last sheet, control the conveying rollers to convey a following sheet to the processing tray, when the controller determines the sheet conveyed to the processing tray is the last sheet, control the horizontal alignment section to realign the last sheet at the first alignment position; and the controller is further configured to determine whether a number of sheets which are loaded on the processing tray is a predetermined number, when the controller determines the number of sheets which are loaded on the processing tray is the predetermined number, control the horizontal alignment section to realign the sheet at the first alignment position, when the controller determines the number of sheets which are loaded on the processing tray is not predetermined number, control the conveying roller to discharge the sheet to the discharge tray. a processing tray configured to load a sheet; a vertical alignment section that is fixedly attached to a rotational shaft, configured to align the sheet on the processing tray in a sheet conveying direction by rotating around the rotational shaft; a horizontal alignment section configured to align the sheet in a sheet width direction orthogonal to the sheet conveying direction, by sandwiching with a first alignment member and a second alignment member; conveying rollers (223a, 223b) configured to convey a sheet; a discharge tray; and a controller configured to control the horizontal alignment section to shift the sheet to a first alignment position a predetermined distance from the center of the processing tray in the sheet width direction, control the horizontal alignment section to align the sheet in the sheet width direction at the first alignment position, control the vertical alignment section to align the sheet in the conveying direction at the first alignment position, and control the horizontal alignment section to realign the sheet at the first alignment position; characterized in that the controller is further configured to determine whether a sheet conveyed to the processing tray is a last sheet, when the controller determines the sheet conveyed to the processing tray is not the last sheet, control the conveying rollers to convey a following sheet to the processing tray, when the controller determines the sheet conveyed to the processing tray is the last sheet, control the horizontal alignment section to realign the last sheet at the first alignment position; and the controller is further configured to determine whether a number of sheets which are loaded on the processing tray is a predetermined number, when the controller determines the number of sheets which are loaded on the processing tray is the predetermined number, control the horizontal alignment section to realign the sheet at the first alignment position, when the controller determines the number of sheets which are loaded on the processing tray is not predetermined number, control the conveying roller to discharge the sheet to the discharge tray.