A sheet stacking device (77) is provided with a sheet discharge platform (76) on which image forming sheets (P), which have been subjected to heating and drying processing after images are formed using ink, are stacked; and a nozzle (162) that blows air to a side surface of a sheet bundle in a horizontal direction. Air is blown to the sheet bundle stacked on the sheet discharge platform (76) while the height of the sheet discharge platform (76) is adjusted by a sheet discharge platform raising and lowering device (100) so that a gap is formed between the sheets in the vertically middle portion of the sheet bundle. According to the sheet stacking device (77), a plurality of image forming sheets (P) can be efficiently cooled on the sheet discharge platform. Accordingly, the occurrence of blocking can be suppressed.