A sheet processing device can prevent a folded sheet bundle from being opened upon accumulation to prevent a reduction in accumulation amount thereof. The sheet processing device includes a pair of pressing rollers pressing a folding loop of a folded sheet bundle in a thickness direction, a support unit mounted to a device frame and supporting a plurality of rows of the pressing roller pairs such that intervals therebetween in the row direction are reduced stepwise, and a gripper member that makes the folding loop of the folded sheet bundle pass through the support unit in a direction that the intervals of the plurality of rows of pressing roller pairs in the row direction are reduced stepwise. The folded sheet bundle passes through the support unit by the gripper member to cause the folding loop to be sequentially pressed by the plurality of rows of pressing roller pairs.