A disclosed sheet aligning device includes a sheet conveyance path; a detecting unit configured to detect a side edge of a sheet being conveyed in the sheet conveyance path; a stopper provided on an upstream side of the detecting unit and configured to open/close in such a manner as to allow/prevent passage of the sheet through the sheet conveyance path and to position a leading edge of the sheet being conveyed in the sheet conveyance path; a first conveying unit provided on an upstream side of the stopper, the first conveying unit including a pair of first rollers configured to come in contact with/separate from each other; a second conveying unit provided on an upstream side of the first conveying unit, the second conveying unit including a pair of second rollers configured to come in contact with/separate from each other; and a horizontal movement unit configured to move the pair of first rollers in an axial direction of the first rollers based on a detection result output by the detecting unit.