A post-processing device includes a device main body, a processing tray, a placed sheet processing part, a stack tray, a detecting part, a stack tray driving part and a sheet conveying part. The processing part carries out post-process to the sheet placed on the processing tray. The stack tray stacks the sheet ejected from a sheet ejection port. The detecting part detects whether or not the stack tray or the sheet on the stack tray is positioned at a detected position below the sheet ejection port. When the sheet is ejected without the post-process, in a detectable case, the conveying part ejects the sheet without placing on the processing tray and, in an undetectable case, the conveying part places the sheet on the processing tray without ejecting to the stack tray until it becomes detectable, and then, ejects the sheet on the processing tray after it becomes detectable.