An attachment structure in a writing instrument, includes: a first component; and a second component configured to be fitted into a fitting hole provided at an end of the first component, the second component including: a small-diameter portion; and a large-diameter portion disposed in front of the small-diameter portion; and a first outer step portion disposed between the small-diameter portion and the large-diameter portion, the fitting hole including: a small-diameter hole portion; and an inner step portion disposed in front of the small-diameter hole portion, the small-diameter portion being fitted into or loosely fitted into the small-diameter hole portion, the first outer step portion coming into contact with the inner step portion, the small-diameter hole portion includes a barb portion protruding from the inner face of the small-diameter hole portion, the barb portion being deformable by contact with the small-diameter portion when the second component is fitted.

A drawing material container cartridge according to one embodiment includes a cartridge body including a tubular portion that accommodates a drawing material, and being configured to be detachably mounted in a container body by being inserted into the container body along an axial direction that is a direction in which an axis of the tubular portion extends; and a cartridge cap that is mounted on the cartridge body by inserting the tubular portion along the axial direction. The cartridge body includes a first locking portion that locks to the container body in the axial direction when the cartridge body is mounted in the container body. The cartridge cap includes a second locking portion that locks to the first locking portion in the axial direction when the cartridge cap is mounted on the cartridge body.

A drawing material container cartridge according to one embodiment includes a cartridge body including a tubular portion that accommodates a drawing material, and being configured to be detachably mounted in a container body by being inserted into the container body along an axial direction that is a direction in which an axis of the tubular portion extends; and a cartridge cap that is mounted on the cartridge body by inserting the tubular portion along the axial direction. The cartridge body includes a first locking portion that locks to the container body in the axial direction when the cartridge body is mounted in the container body. The cartridge cap includes a second locking portion that locks to the first locking portion in the axial direction when the cartridge cap is mounted on the cartridge body.

A pen includes a pen tube, a liquid guide accommodated in the pen tube, a tail sleeve detachably connected to the pen tube, and a liquid cartridge. The liquid cartridge is accommodated in the tail sleeve and extends into the pen tube. The liquid cartridge includes a container and a plug structure. The container has an opening. The plug structure is disposed in the container and includes a cover portion, an elastic connection portion, and an annular wall portion. The elastic connection portion is connected to the cover portion and has a hollow portion. The annular wall portion is connected to the elastic connection portion and surrounds the cover portion. An annular gap is formed between the cover portion and the annular wall portion and communicated with the hollow portion. The liquid guide is pressed against the cover portion to open the annular gap.