A sheet processor subjecting a sheet having been conveyed forward to processing along a direction perpendicular to a conveyance direction of the sheet, includes: a processing unit performing the processing; and a receiving unit receiving the processing unit therein in a state capable of perfecting the processing on the sheet, and the processing unit includes a first processing tool 4A and a second processing tool 4B disposed to vertically oppose each other with a conveyance surface of the sheet disposed therebetween, and the receiving unit includes at least one receiver that removably receives the first processing tool 4A and the second processing tool 4B in the state capable of performing the processing on the sheet, with arbitrarily selected one of the first processing tool 4A and the second processing tool 4B disposed above the conveyance surface, and with arbitrarily selected another of the first processing tool 4A and the second processing tool 4B disposed below the conveyance surface.