Image forming apparatus

A sheet processing apparatus provided with sheet stacking means supported in vertically movable manner and adapted to support sheets discharged from the main body of said sheet processing apparatus, and elevate/lower means for elevating or lowering said sheet stacking means, comprising:

• detection means for outputting a detection signal by detecting an uppermost one among the sheets discharged in succession and stacked on said sheet stacking means;
• and
• control means for controlling said elevate/lower means so as to lower said sheet stacking means in response to the detection signal inputted from said detection means;
• wherein, said control means after the input of the detection signal from said detection means, if the input of said detection signal is not terminated even after lowering of said sheet stacking means by a predetermined amount, executes a serial control of lowering said sheet stacking means until the input of said detection signal is terminated, then elevating said sheet stacking means until said detection signal is inputted, and lowering said sheet stacking means until said detection signal is terminated.