The present invention is to provide a sheet aligning apparatus that is capable of detecting misalignment. A control portion of a post-processing apparatus causes a front aligning member and a rear aligning member to be moved to an aligning position to align sheets conveyed to a processing tray, causes the front aligning member to be moved from the aligning position to a detecting position, and determines whether an electrostatic capacitance sensor detects misalignment (sheet shifting from a sheet bundle). When it is determined that the sensor detects misalignment, the front aligning member is moved from the detecting position to the aligning position so that sheets are realigned.