Abstract: A method of sealing an opening portion of a packaging container by utilizing fusion bonding of the base material, together with an apparatus and method for inspecting the sealed portion. The container material has a welding layer as the innermost layer. The welding layer is melted at a seal portion defined along the edge of the opening portion of the container. A part of the melt from the welding layer is allowed to flow out from the edge of the opening portion, thereby sealing the seal portion and also forming an efflux layer outside the opening portion by the melt flowing out and solidified. An image of at least a part of the sealed portion and its vicinities is taken. At least a part of the image is defined as an inspection region. The image of the inspection region is subjected to predetermined image processing to calculate data concerning the efflux layer formed outside the sealed portion.