In a sheet post-finishing apparatus, a stapling device conducts a saddle stitching process on a bundle of sheets, based on an instruction signal; a sheet guide member, mounted on the stapling device, faces a surface of a sheet to be stacked on a sheet stacking section; a stapling device moving section allows the stapling device to move along a line dividing the sheet into two equal portions, perpendicular to a sheet conveyance direction; and a control section controls the stapling device to stand by above a lateral center of the sheet to be conveyed to the stapling device, while the sheets are stacked as the bundle of sheets, and after the bundle of sheets is formed, the control section controls the stapling device to sequentially move to predetermined plural positions, whereby said control section sends the instruction signal to the stapling device which was stopped at the predetermined positions.