Abstract: A folding box of cardboard or similar material, comprising a casing having a longitudinal joint and top and bottom end closures and preferably sealing membranes attached to flaps of said end closures. The longitudinal joint of the folding box is formed as an abutting edge joint which at least partially along a substantial length of the total folding box length has an extension which is not parallel to the side edges of the folding box. Especially in that case where sealing membranes exist, the joint extends the whole way out to the free cut edges of each end flap of the top and bottom end closure, respectively, and the cross section of the folding box is covered by a sealing membrane attached against flaps at each end of the folding box. The longitudinal joint is covered by a strip, placed externally or internally or on both sides of the wall. The strip is of a thin material, preferably of a thickness around 20-50 .mu.m and covers the joint at both sides thereof.