In a cylindrical knife holder rotatable about its longitudinal axis, at least one groove is present. The groove extends in the direction of the longitudinal axis of the knife holder. Inserted exchangeably in this groove is a perforating knife, the perforating edge of which protrudes over the peripheral surface of the knife holder. The perforating knife, which can be clamped in the groove by a releasable clamping device, is supported, at its end lying opposite the perforating edge, on a stop element disposed in the groove. The stop element rests on supporting elements distributed over the length of the groove, which are adjustable in directions running transversely to the longitudinal extent of the groove. The supporting elements define the once set radial position of the perforating knife.