To provide a method for manufacturing a cap that allows a thin plate member to be inserted into a predetermined upper position in a cap main body without deforming of the thin plane member and the cap main body, and prevents the thin plate member from breaking away when reducing pressure of a container.

In a circular flat area 6c of a thin plane member 6 of an inner plug 10 is formed a circular protrusion 50 protruding toward the lower surface. After the thin plate member 6 was insertedly arranged in a predetermined position in the cap main body 4 or at the same time when the plate member is insertedly arranged in the position, the circular protrusion 50 is pressed against a ceiling wall 12 in an axial direction and deformed to be formed into a nearly flat shape so as to increase an outer diameter of the thin plane member 6, thereby holding the inner plug 10 so as not to break away from inside a skirt wall 14, and to move in an axial direction.