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. The sheet member does not separate when a container is brought to reduced pressure. An annular protruding portion protruding on a lower surface side is formed in an annular flat region of a sheet member of an inside plug. After or at the same time that the sheet member is inserted into and installed at a predetermined position within a cap body, the annular protruding portion is pressed axially toward a top panel wall and deformed so as to become close to a flat shape, to enlarge the outer diameter of the sheet member. The inside plug is thereby held to be inseparable from inside a skirt wall and to be axially movable.