Provided are a post-processing device, an image forming apparatus having the post-processing device and an error handling method therefor. A conveyed sheet is temporarily stacked on a pair of slide guides (310, 311), and after post-processing such as stapling is conducted on the sheets, the slide guides (310, 311) are opened to drop down the sheets on a face-down discharge portion (125) on an upper surface of a printer (100). If the number of sheets temporarily stacked on the slide guides (310, 311) exceeds the post-processing possible number of sheets, the slide guides (310, 311) are opened to drop down the sheets on the face-down discharge portion (125). If the size of sheets to be post-processed is unsuitable for the post-processing, a flapper (301) is changed over so that the sheets transported from the printer main body (100) are discharged directly to the face-down discharge portion (125) and stacked thereon.