[Problems to be Solved] There is provided a method for manufacturing a cap, which can insert a sheet member easily into a predetermined position of an upper part inside a cap body, without causing deformation to the sheet member or the cap body, and in which the sheet member does not separate when a container is brought to reduced pressure. [Means to Solve the Problems] An annular protruding portion 50 protruding on a lower surface side is formed in an annular flat region 6c of a sheet member 6 of an inside plug 10. After or at the same time that the sheet member 6 is inserted into and installed at a predetermined position within a cap body 4, the annular protruding portion 50 is pressed axially toward a top panel wall 12 and deformed so as to become close to a flat shape, to enlarge the outer diameter of the sheet member 6. By so doing, the inside plug 10 is held to be inseparable from inside a skirt wall 14 and to be axially movable.