Device for bringing into register a tool mounted on a rotary cylinder for processing products in sheet form, such as a tool for cutting out, creasing, or printing sheets of cardboard in a machine for making and printing corrugated cardboard boxes, comprising a coder (22) providing information representing the theoretical advance of the sheets (10, 11), a coder (23) providing information representing the angle of adjustment given to the tool holder (1), a detector (20) of the passage of a reference mark (15) on the sheet (11) to a point situated upstream of the tool holder, a detector (36) of the theoretical passage of this reference mark at a selected distance slightly downstream of the first detector (20), and a computer (33) supplying correction commands to the correction motor (8) of the tool holder in order that the tool will arrive in phase with the incident sheet (11).