A sheet post-processing device includes a stacker for stacking and storing sheets therein, a feeder for receiving sheets and feeding the sheets toward the stacker, a folder for folding a set of sheets stored in the stacker, a preparatory processor, which is located upstream from the folder, for processing sheets preparatory to folding of a set of sheets, a presser for pressing the set of sheets stored in the stacker at a portion processed by the preparatory processor so as to push the set of sheets into the folder, and a controller for controlling the stacker, the feeder, the folder, the preparatory processor and the presser. The controller controls the preparatory processor not to process a sheet that will be an outermost sheet of a folded set of sheets.