Abstract: A packaging box is assembled from a blank including a trapezoidal locking tab formed adjacent to one end edge of the blank by a cut formed therein. An insert tab extends from the other end edge of the blank and can be inserted into an opening which is formed when the locking tab is folded. When an article or articles are packaged, the insert tab is inserted into the opening from the outside while tilting the locking tab, and when fitted into the opening, the insert tab is urged by gravity of the articles, whereby it is locked in place. In this manner, the packaging box is rigidly maintained in its assembled condition.