A width expanding device (100) comprises two rollers (1, 2, 3,) having complementary contours (15), e.g. helical contours, which mutually engage when the rollers (1, 2, 3) rotate. The product web (10) is taken through the expansion slot (12, 13) formed between the rollers (1, 2 or 1, 3) and is expanded in accordance with the depth of engagement of the contours (15) between two adjacent rises in the contours (5) across the width of the product web (10). The specific local width expansion is thus uniformly achieved over the entire width of the product web. The essential feature for the occurence of the effect is a pressure roller (7) adjacent to the expansion slots (12, 13) and bearing on the rollers (2, 3) which clamps the product web (10) between itself and the peaks of the reliefs (15) and secures it across its width.