A writing instrument comprising: a writing instrument body provided with a pen point at a front end of a barrel; and an ink cartridge connected to a rear end side of the writing instrument body, wherein the ink cartridge includes a bottomed cylindrical container with an opened front end and a closed rear end and a plug fitted into an inner circumferential surface of an opening of the container, the writing instrument body includes a projection that opens the ink cartridge when inserted into the opening of the container to press the plug rearward and a connecting portion that is formed separately from the projection and also surrounding the projection, and the writing instrument body and the ink cartridge are in a connected condition by air tight fitting the opening of the container in the connecting portion, and wherein a plurality of longitudinal channels are formed in an outer periphery of the projection, wherein at least one of the channels becomes an ink channel and at least one of the other channels becomes an air intake path, wherein an intermediate member is press fitted in the barrel in a cylindrical shape of the writing instrument body and the connecting portion is provided behind a fitting portion of the intermediate member, and further characterised in that a locking portion is formed on the intermediate member and also a locked portion is formed on an inner circumferential surface of the barrel, wherein the locking portion and the locked portion are axially extending ribs, and when a rotating force is applied to the connecting portion, the locking portion makes contact with and locks the locked portion to restrict rotation of the intermediate member, wherein the connected condition is achieved by air tight fitting an outer peripheral surface of the opening of the container in an inner side surface of the connecting portion, wherein the connecting portion is an axially extending cylindrical shape, and/or wherein an air tight annular rib is formed on the inner side surface of the connecting portion and/or on the outer peripheral surface of the opening of the container. a writing instrument body provided with a pen point at a front end of a barrel; and an ink cartridge connected to a rear end side of the writing instrument body, wherein the ink cartridge includes a bottomed cylindrical container with an opened front end and a closed rear end and a plug fitted into an inner circumferential surface of an opening of the container, the writing instrument body includes a projection that opens the ink cartridge when inserted into the opening of the container to press the plug rearward and a connecting portion that is formed separately from the projection and also surrounding the projection, and the writing instrument body and the ink cartridge are in a connected condition by air tight fitting the opening of the container in the connecting portion, and wherein a plurality of longitudinal channels are formed in an outer periphery of the projection, wherein at least one of the channels becomes an ink channel and at least one of the other channels becomes an air intake path, wherein an intermediate member is press fitted in the barrel in a cylindrical shape of the writing instrument body and the connecting portion is provided behind a fitting portion of the intermediate member, and further characterised in that a locking portion is formed on the intermediate member and also a locked portion is formed on an inner circumferential surface of the barrel, wherein the locking portion and the locked portion are axially extending ribs, and when a rotating force is applied to the connecting portion, the locking portion makes contact with and locks the locked portion to restrict rotation of the intermediate member, wherein the connected condition is achieved by air tight fitting an outer peripheral surface of the opening of the container in an inner side surface of the connecting portion, wherein the connecting portion is an axially extending cylindrical shape, and/or wherein an air tight annular rib is formed on the inner side surface of the connecting portion and/or on the outer peripheral surface of the opening of the container.