To provide a stack tray causing no jam by properly controlling it even when a delivered sheet contacts with or remarkably approaches an image reader.
This sheet processing unit has the stack tray 411 for loading the sheet or a sheet bundle, a bundle delivering belt for delivering sheets, a motor for lifting the stack tray 411, a sensor for detecting the delivery directional rear end height of an upper surface of the delivered sheet, a detecting sensor arranged above the static tray 411, and detecting that the uppermost position of the delivered sheet reaches a prescribed position, and a finisher control part for controlling operation of respective means, and is characterized by lowering the stack tray 411 on the basis of information on a stack tray paper height sensor and a sheet upper limit detecting sensor.
ISOBE YOSHINORI
MATSUMOTO YUZO
