To stack each sheet in printing two sheets simultaneously with a simple constitution.

This sheet stacking apparatus is capable of two sheets simultaneous printing, and has a disk unit including four substantially identical rotatable disks (100, 102, 104, 106). Two of the four disks (100, 102) receive portions of a first sheet (120) and the remaining two disks (104, 106) receive portions of a second sheet (122). The apparatus further has a separator 110 selectively extendable or retractable, which separates the first sheet and the second sheet into two separate stacks. When it deals with a big single sheet, the sheet is turned over by four disks to be stacked. At this time, the separator 110 is retracted.