A post-processing device for applying a post-process to one or more sheets outputted by a device of a preceding stage, a printer for example, is provided. The post-processing device includes: an alignment mechanism for applying an alignment process for arranging a current sheet bundle; an output mechanism for outputting the current sheet bundle; a prestacking mechanism; and a control unit. The control unit performs a control in such a manner that at least a part of operation of the alignment mechanism or of operation of the output mechanism is switched from a normal operation mode for operation at a normal operation speed to a high-speed operation mode for operation at an operation speed greater than the normal operation speed when a time to completely output the current sheet bundle is later than a time to completely perform prestacking the next sheet bundle.