To bring a state where rear ends of sheets discharged onto the sheet support surface of a stack tray are rested against a rear end support member, into a state where the sheets are bundled and stacked on the horizontal sheet support surface overlappingly in a vertical direction.

This paper discharge tray device 1 comprises the stack tray 2 rotatably between a tilted attitude tilted upward as the sheet support surface extends forward in the sheet discharge direction and a horizontal attitude, a lowering device M1+13 to 15 lowering the stack tray 2 according to a sheet loaded amount, a stack tray rotating device M2+11+16 to 19 rotating the sheet support surface from the tilted attitude to the horizontal attitude in the moved state of the stack tray 2 to the lower end position, and a rear end support member moving device M3+21+24+25 moving forward the rear end of the sheets on the stack tray 2 by moving forward the lower end part of the sheet rear end support member in the sheet discharge direction.