A gas-pressurized writing instrument simply constructed so that a compressed gas will not escape to the outside by providing a writing point assembly (10) at the front end (14a) of a tube member (14), storing an ink (12) to be fed to the writing point assembly (10) in the tube member (14), encasing at least part of the tube member (10) by a casing cylinder (18) formed of a material that presents low gas permeability or gas impermeability so as to form a space (16) communicating with the rear opening of tube member (14), sealing a compressed gas inside the space (16) so that the compressed gas will push the ink out to writing point assembly (10) to secure the writing performance, and covering the outer periphery of the casing cylinder (18) with an exterior member having thermal resistance or/and moisture resistance greater than that of the casing cylinder so that the casing cylinder can present low gas permeability or gas impermeability.