To provide a fitting method for a closable opening device, which does not cause the delamination of an openable area laminate and prevents a liquid content from infiltrating into the section of a container wall, and a packaging container using the opening device.

The closable opening device comprises a frame forming an outlet, a cap fitted on the frame, and a movable ring housed in the frame and moved in reverse to the cap, and the fitting method for the opening device and the packaging container using the same are provided. The frame comprises a flange, the cylindrical outlet, and a central board, which is divided into a plurality of nails connected to the outlet via hinges and is depressed to be widened by the movable ring upon opening the cap. A joining zone is formed between the bottom of the flange and the peripheral surface of the openable area so that the frame is joined to the packaging container at the periphery of the openable area.