A sheet sorter includes an array of a plurality of sorting trays which are mounted on a sorter body and arranged in a vertical direction and a general-purpose tray which is mounted on the sorter body and is larger than each of the sorting trays in capacity so that it accommodates a larger number of sheets than each of the sorting trays. A tray drive mechanism provided in the sorter body moves up and down the sorting trays and the general-purpose tray so that sheets transferred by a transfer mechanism provided in the sorter body are selectively discharged onto the sorting trays or the general-purpose tray. The array of the sorting trays is positioned above the general-purpose tray. The sorting tray array and the general-purpose tray are normally held in a position where the sheet discharge port of the sheet transfer mechanism is positioned between the lowermost tray of the sorting tray array and the general-purpose tray as seen in a vertical direction.