Sheet processing apparatus and image forming apparatus

A sheet processing apparatus comprising:

• a conveyor to convey a sheet along a main conveyance passage in a conveyance direction;
• a first folding roller;
• a second folding roller, facing the first folding roller across the main conveyance passage, to convey the sheet by a first conveyance amount;
• a return conveyance passage having a curvature, the return conveyance passage being:
• branched from the main conveyance passage at a branching position upstream from the first folding roller in the conveyance direction;
• and
• merged to the main conveyance passage at a merging position upstream from the branching position in the conveyance direction;
• and
• a third folding roller, facing the second folding roller across the return conveyance passage, to convey the sheet by a second conveyance amount smaller than the first conveyance amount of the second folding roller,
• wherein the second folding roller disposed on an outer peripheral side of the return conveyance passage, and
• the third folding roller disposed on an inner peripheral side of the return conveyance passage.