A prefolding unit for prefolding a blank; the prefolding unit comprising: a packing conveyor with a packing pocket which is fed along a packing path; an insertion station where the packing pocket of the packing conveyor is stopped to receive the blank; a transfer device, which transfers the blank, in a transfer direction, into the packing pocket of the packing conveyor stopped at the insertion station, so that at least one end (4; 5) of the blank projects from the packing pocket; an output station where the partly folded blank is expelled from the packing pocket; at least one work station (S3, S4) located along the packing path, between the insertion station and the output station, and which comprises an operating member (14, 15) for operating on the blank; and at least one guide device, which is located in a fixed position between the insertion station and the work station (S3, S4), is perpendicular to the packing path, and holds the end (4; 5) of the blank, projecting from the packing pocket, in a predetermined position as the packing conveyor moves the packing pocket out of the insertion station to the work station (S3, S4); the prefolding unit being characterized in that: the guide device comprises a top guide member and bottom guide member, which are located above and below the packing path, and define a gap through which the end (4; 5) of the blank projecting from the packing pocket passes; and facing the insertion station, the top and bottom guide members (17, 18) are triangular, so an inlet portion of the gap decreases gradually in size to act as a lead-in for the end (4; 5) of the blank projecting from the packing pocket. a packing conveyor with a packing pocket which is fed along a packing path; an insertion station where the packing pocket of the packing conveyor is stopped to receive the blank; a transfer device, which transfers the blank, in a transfer direction, into the packing pocket of the packing conveyor stopped at the insertion station, so that at least one end (4; 5) of the blank projects from the packing pocket; an output station where the partly folded blank is expelled from the packing pocket; at least one work station (S3, S4) located along the packing path, between the insertion station and the output station, and which comprises an operating member (14, 15) for operating on the blank; and at least one guide device, which is located in a fixed position between the insertion station and the work station (S3, S4), is perpendicular to the packing path, and holds the end (4; 5) of the blank, projecting from the packing pocket, in a predetermined position as the packing conveyor moves the packing pocket out of the insertion station to the work station (S3, S4); the prefolding unit being characterized in that: the guide device comprises a top guide member and bottom guide member, which are located above and below the packing path, and define a gap through which the end (4; 5) of the blank projecting from the packing pocket passes; and facing the insertion station, the top and bottom guide members (17, 18) are triangular, so an inlet portion of the gap decreases gradually in size to act as a lead-in for the end (4; 5) of the blank projecting from the packing pocket. the guide device comprises a top guide member and bottom guide member, which are located above and below the packing path, and define a gap through which the end (4; 5) of the blank projecting from the packing pocket passes; and facing the insertion station, the top and bottom guide members (17, 18) are triangular, so an inlet portion of the gap decreases gradually in size to act as a lead-in for the end (4; 5) of the blank projecting from the packing pocket.