Abstract: A strip of paperboard material having a thickness of 170 to 600 .mu.m and a multiplicity of fold lines formed widthwise thereof at a predetermined spacing and U-shaped in cross section is caused to travel as reeved around two prefolding rolls, 5 to 30 mm in diameter, from different directions at a contact angle of 40 to 270 degrees to bring the projecting side of the fold lines into contact with the peripheral suface of one of the rolls and the indented side of the fold lines with the peripheral surface of the other roll. This method imparts increased foldability to the fold lines.