To provide a means capable of easily preventing a reduction in carbon dioxide with time in a bottle product filled with a content mainly composed of soda water, without change in layer structure of the bottle or the like or without installation of a dedicated facility.

A cap body 1 for sealing a mouth tube part of a synthetic resin bottle body includes a package body 21 disposed inside a cylindrical cap body 1 with a closed top, the package body being formed by sealing a carbon dioxide generator which generates carbon dioxide by the action of moisture with a packaging material at least part of which is formed from a synthetic resin permeable film. The permeable film has a steam permeation coefficient of 0.1 g mm/(m² day) or more.