A sheet processing device includes: a conveying module that conveys a folded sheet; and a pressing module that presses a folded part of the folded sheet by rotating about a direction orthogonal to a sheet conveying direction of the conveying module as a rotation axis. The pressing module includes a projecting part arranged in a certain range in a direction of the rotation axis along a circumferential surface about the rotation axis. The projecting part is formed to be symmetric with respect to a middle part of the rotation axis in the direction of the rotation axis, and the projecting part arranged on one side from the middle part along the direction of the rotation axis are formed such that a position of the projecting part in a rotational direction of the circumferential surface varies along the direction of the rotation axis.