Sheet stacking apparatus and image forming system

A sheet stacking apparatus comprising:

• a stacking portion configured to stack a sheet;
• a conveyance portion configured to convey the sheet toward the stacking portion;
• a belt member having elasticity and configured to move an uppermost sheet stacked on the stacking portion in a sheet conveyance direction;
• an abutment portion against which a leading edge of the sheet conveyed by the belt member in the sheet conveyance direction abuts;
• a driving rotary member being in contact with one surface of the belt member, and configured to rotate the belt member, and
• a driven rotary member forming a nip portion in which the belt member is nipped by the driving rotary member and the driven rotary member, being in contact with another surface of the belt member in the nip portion, and configured to be rotated by the belt member,
• wherein the driving rotary member and the driven rotary member are disposed such that when viewed in a rotation-axis direction of the driving rotary member, a downstream segment, in a moving direction of the belt member, of a tangential line that is in contact with the belt member in the nip portion and extends from the nip portion is located in a region downstream of the nip portion in the sheet conveyance direction, and in a region from the nip portion toward the stacking portion.