in a plastic sheet having a plurality of creasing lines , each creasing line is formed of a groove having a narrow bottom surface and a pair of opposed side surfaces each slanting at a certain angle ; and a plurality of transverse grooves are disposed at a certain pitch in the longitudinal direction of the groove . each transverse groove is formed by two or more curved projections to have at least one narrow bottom surface extending in a direction substantially perpendicular to the longitudinal direction of the groove .