To execute accurate paper position control when another post-processing is performed on a paper on which a folding line is formed.

In post-processing in a post-processor 300 having a position control member for controlling a paper position and a controller 301 for controlling a control state of the position control member, a position of the paper in a direction perpendicular to a folding line is controlled to be changed depending on whether or not the folding line is formed on the paper. The position control member serves as an alignment member which changes the paper position from an end portion of the paper for alignment and exerts control so as to change the paper position in aligning the end portion of the paper parallel with a folding line by the alignment member depending on whether or not the folding line is formed on the paper.