To provide a sheet processor for carrying out middle binding without breaking a sheet bundle.

In the sheet processor, a width center position of a folding processing route 60 of a first stack tray 50, a width center position for conforming the sheet bundle by a right-and-left width regulating plate 66, and a width center position for binding the sheet bundle with a middle stapler unit 67 correspond with each other. When the sheet bundle is moved from a processing tray 14 to the middle stapler unit 67, it need not be moved to the width direction. Accordingly, the width conformity is rarely disarrayed by breaking the sheet bundle.