The invention is to improve the inspection accuracy and to detect all defects including a top line, a top bubble, a defective top flatness, and a top dimple only by means of a single inspection device. [MEANS FOR SOLVING PROBLEMS] When there is one region where the edge signal exceeds an edge signal threshold (EL), the address generated by adding a predetermined offset value (OFS) to the address (inspection gate edge address GE) where the edge signal first reaches the edge signal threshold (EL) is used as the inspection gate start position address (GES); when there are two regions where the edge signal exceeds the edge signal threshold (EL), the address generated by adding the predetermined offset value (OFS) to the address (inspection gate edge address GE) in the second region where the edge signal first reaches the edge signal threshold (EL) is used as the inspection gate start position address (GES). Therefore, an inspection gate which is the region to be inspected can be properly made, and top line/bubble inspection of the mouth top can be carried out accurately without being influenced by various types of noise.