A sheet post-processing device comprising: a housing which includes a paper feed port and a paper discharge port; a main transport path which transports a sheet from the paper feed port to the paper discharge port; a paper discharge tray which is provided outside the housing and which receives the sheet discharged from the paper discharge port; a paper discharge roller pair which is provided at a downstream-side end portion of the main transport path; a supply roller pair which is arranged on an upstream side with respect to the paper discharge roller pair of the main transport path; a post-processing transport path whose one end portion is connected to the downstream-side end portion of the main transport path and whose other end portion is connected to a post-processing portion that performs post-processing on a stack of sheets; a post-processing tray which is arranged along the post-processing transport path and on which the sheets supplied by the supply roller pair are sequentially stacked; and a reference plate portion which is provided in the post-processing portion and which makes contact with a back edge of the sheets supplied onto the post-processing tray to align the sheets, wherein the paper discharge roller pair comprises a driven roller and a drive roller which is arranged opposite the driven roller and which can be moved between a nip position where a nip portion is formed between the drive roller and the driven roller and a maximum separate position a predetermined distance separate from the driven roller, the post-processing device comprises a drive controller which controls an operation of rotation of the drive roller and an operation of separating from and making contact with the driven roller by the drive roller, characterised in that the drive controller separates the drive roller from the driven roller to place the drive roller on standby in a predetermined standby position when the sheet is supplied and transported by the supply roller pair onto the post-processing tray, moves the drive roller to the nip position after the back edge of the sheet is passed through the supply roller pair and rotates the drive roller in a reverse rotation direction so as to bring the back end of the sheets into contact with the reference plate portion and to align the sheets, and the standby position of the drive roller when a first sheet in the stack of sheets to be subjected to the post-processing by the post-processing portion is transported onto the post-processing tray is a position between the maximum separate position and the nip position. a housing which includes a paper feed port and a paper discharge port; a main transport path which transports a sheet from the paper feed port to the paper discharge port; a paper discharge tray which is provided outside the housing and which receives the sheet discharged from the paper discharge port; a paper discharge roller pair which is provided at a downstream-side end portion of the main transport path; a supply roller pair which is arranged on an upstream side with respect to the paper discharge roller pair of the main transport path; a post-processing transport path whose one end portion is connected to the downstream-side end portion of the main transport path and whose other end portion is connected to a post-processing portion that performs post-processing on a stack of sheets; a post-processing tray which is arranged along the post-processing transport path and on which the sheets supplied by the supply roller pair are sequentially stacked; and a reference plate portion which is provided in the post-processing portion and which makes contact with a back edge of the sheets supplied onto the post-processing tray to align the sheets, wherein the paper discharge roller pair comprises a driven roller and a drive roller which is arranged opposite the driven roller and which can be moved between a nip position where a nip portion is formed between the drive roller and the driven roller and a maximum separate position a predetermined distance separate from the driven roller, the post-processing device comprises a drive controller which controls an operation of rotation of the drive roller and an operation of separating from and making contact with the driven roller by the drive roller, characterised in that the drive controller separates the drive roller from the driven roller to place the drive roller on standby in a predetermined standby position when the sheet is supplied and transported by the supply roller pair onto the post-processing tray, moves the drive roller to the nip position after the back edge of the sheet is passed through the supply roller pair and rotates the drive roller in a reverse rotation direction so as to bring the back end of the sheets into contact with the reference plate portion and to align the sheets, and the standby position of the drive roller when a first sheet in the stack of sheets to be subjected to the post-processing by the post-processing portion is transported onto the post-processing tray is a position between the maximum separate position and the nip position.