Sheet processing apparatus and image forming apparatus

An image forming apparatus comprising:

• an image forming portion which forms an image on a sheet;
• and
• a sheet processing apparatus on which a sheet bundle is stacked, the sheet bundle being formed by arranging sheets into a bundle and folding the bundle,
• wherein said sheet processing apparatus includes:
• (a) a sheet discharge portion which discharges a folded sheet bundle with a folded portion being the leading edge;
• (b) a sheet stack portion which stacks the folded sheet bundle discharged from said sheet discharge portion, said sheet stack portion including a sheet moving member which moves the stacked sheet bundle in a sheet discharge direction and in an opposite direction to the sheet discharge direction;
• and
• (c) a controller which controls said sheet moving member to move a preceding sheet bundle stacked on said sheet stack portion so that the preceding sheet bundle is stacked while being moved in the sheet discharge direction and to move the preceding sheet bundle stacked on said sheet stack portion in a direction opposite to the sheet discharge direction before a folded portion of a succeeding sheet bundle contacts the preceding sheet bundle so that a contact portion between the preceding sheet bundle and the succeeding sheet bundle is on an upper surface of the preceding sheet bundle.