A packaging box includes a body part, flaps and a buffer. The body part has a rectangular bottom plate and side plates erected from four sides of the bottom plate, and is provided with an opening at an upper face. The flaps are connected to upper edges of the side plates via fold lines to close the opening by being folded inside. The buffer is interposed between the body part and a packaged article. The buffer has an engaging groove formed so as to grip the side plate and the flap folded up the outside of the side plate by being folded back outside along the fold line.