To arrange and bind sheets.

The saddle stitch bookbinding device 23 includes: a creasing unit 4 that creases a middle portion of a sheet orthogonally to a sheet conveying direction; a control roller pair 31 that conveys the sheet creased by the creasing unit; a stopper that matches the ends in the sheet conveying direction of a plurality of creased sheets conveyed by the control roller pair; a saddle stitch stapler that binds a portion being the spine of the sheet matched by the stopper when the sheet is bent; and a bending mechanism that bends the creased portion of the sheet. The control roller pair flattens the creased portion while conveying the sheet. Therefore, the sheet is matched by the stopper, to be easily superimposed, arranged and bound.