To effectively correct a sheet that is transported while being skewed, a post-processing apparatus B provided with a processing tray 25 that temporarily stores a sheet to perform post-processing and a stack tray 30 that loads sheets subjected to the post-processing performs preliminary alignment with one of an alignment mechanism 35 on the processing tray 25 side and an alignment mechanism 38 on the stack tray 30 side corresponding to size information of a sheet in transporting the sheet from the stack tray 30 side toward the processing tray 25 for post-processing in a post-processing device 31, and takes in the sheet toward a regulation stopper 29 with a take-in roller 33. Then, after aligning the sheet with the alignment mechanism 35, the apparatus performs the post-processing on the sheet with the sheet post-processing device 31 to transport to the stack tray 30.