Provided is a writing instrument, having high cartridge replaceability and excellent convenience, that is capable of suppressing inappropriate ink leakage that easily occurs when a used cartridge container is pulled out even when the writing instrument uses a replacement ink cartridge with a plug to be press opened. A writing instrument includes: a replacement ink cartridge including a bottomed cylindrical container that has an opened front end and a closed rear end and a plug fitted into an inner circumferential surface of an opening of the container; and a projection inserted into the opening of the container to press the plug rearward, thereby opening the plug of the ink cartridge, wherein the projection and the plug are fitted when the plug is press opened, and when the cartridge is replaced, the fitting between the projection and the plug is released and the plug is locked again in the opening of the container.