A sheet processing apparatus which can be reduced in size and suppress cost increase by preventing sheet buckling and jamming. Sheets stacked on an insert tray of a sheet processing apparatus are fed on a conveying path in a first sheet feed mode in which sheets are fed after being inverted by an inversion path or a second sheet feed mode in which sheets are fed without passing through the inversion path. A pair of conveying rollers conveys the fed sheets and the sheets with images formed thereon. The conveyed sheets are stacked on a processing tray, and a stapler performs predetermined processing on the stacked sheets. It is determined whether predetermined information is included in sheet information about the sheets stacked on the insert tray, and if the predetermined information is included in the set sheet information, a predetermined message is displayed on a display panel.