Disclosed is a cartridge cap capable of preventing air that is trapped when sealing the opening of a cylindrical cartridge filled with a medicinal solution from remaining in the cylindrical cartridge. Air which is enclosed when sealing the opening of a cylindrical cartridge (2) filled with a medicinal solution escapes to the outside of the cylindrical cartridge (2) via air relief passages (AR) which are formed around the inner peripheral surface of a cylindrical wall (1B), and comprise the gaps between the outer peripheral surface of the lip (2A) of the cylindrical cartridge (2) and the internal peripheral surfaces of the cylindrical wall (1B) of the cartridge cap (1) where four raised receiving sections (1F) are not formed. It is therefore possible to prevent air remaining inside the cylindrical cartridge (2).
