The invention provides a sheet storing apparatus, a post-processing apparatus and an image forming system having the same. In any of conveyance modes in which a sheet is conveyed from the sheet discharging port to a processing tray and a sheet fed to a discharging port is conveyed toward a stack tray, the sheet can be reliably conveyed and ink friction and damage to an image sheet can be prevented. In the provided apparatus, an upper roller to be engaged with a sheet upper face and a lower roller to be engaged with a sheet lower face are arranged at a sheet discharging port in a manner capable of being pressure-contacted and being separated, the upper roller is formed with a large-diameter soft roll face and a small-diameter hard roll face, and a pressurization force of roller lifting-lowering means with which the upper roller is pressure-contacted to and is separated from the lower roller is switched to be high or low.