A writing instrument includes: a writing instrument body and an ink cartridge. The ink cartridge includes a bottomed cylindrical container with an opened front 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 to press the plug rearward and an air tight connecting portion, formed separately from the projection, that surrounds the projection. The opening is divided into a plurality of openings by air tight fitting the opening in the air tight connecting portion. A plurality of longitudinal channels are formed on an outer periphery of the projection. At least one of the plurality of channels serves as an ink channel and at least one of the other channels serves as an air intake path.