The present invention provides an outlet cap adapted to removably attached to the gas outlet of a gas discharge passageway in an on-off valve provided on a toxic gas containing container, the outlet cap comprising a bottom wall, a peripheral wall extending from the bottom wall and a gasket located on an inner surface of the bottom wall for sealing the gas outlet, the gasket including a gas absorbing means which is made of a material which does not chemically react with the toxic gas contained in the toxic gas container to degrade the sealing property of the gasket, the toxic-gas absorbing means being exposed to the gas discharge passageway when the outlet cap is attached to the gas outlet.