Patent ID: 8011649

Claim:
A sheet processing comprising: a first detector configured to detect a leading edge of a sheet conveyed in a conveying direction; a second detector provided on a downstream side of the first detector in the sheet conveying direction to detect the leading edge of the sheet conveyed; a third detector provided on an upstream side of the first detector in the sheet conveying direction to detect the leading edge of the sheet conveyed; a punching portion, in the downstream side of the first detector in the sheet conveying direction, configured to move in a sheet width direction crossing the sheet conveying direction and perform a punching process for the sheet; an edge detector configured to move in the width direction together with the punching portion and detect the edge of the sheet in the width direction; and a controller configured to judge, on the basis of at least one of the information of a conveying speed of the sheet and a sheet length in the conveying direction, when the edge detector starts movement in the width direction after any one of the first, second, and third detectors detects the leading edge of the sheet, that the edge detector can detect the edge of the sheet, to select, as a result of the judgment, among the first, second, and third detectors, the detector positioned on the most downstream side in the conveying direction which can be used, and to permit the edge detector to start movement to detect the edge of the sheet in the width direction when the selected detector among the first, second, and third detectors detects the leading edge of the sheet conveyed.