Writing instrument and ink cartridge

A writing instrument, comprising:

• a writing instrument body provided with a pen point to be a writing tip end at a front end of a barrel;
• and a replacement ink cartridge provided with a tubular container having an opened front end and a closed rear end, wherein
• the ink cartridge has a plug fitted in an inner circumferential surface of an opening of the container and blocking the opening,
• the writing instrument body has a projection pressing the plug rearward when inserted into the opening of the container to release fitting between the plug and the inner circumferential surface of the opening of the container, and
• any one of the plug and the projection is provided with a male fitting portion and the other is provided with a female fitting portion, wherein
• the fitting between the plug and the inner circumferential surface of the opening of the container is released in a procedure of attaching the ink cartridge to the writing instrument body,
• the plug is fitted again in the inner circumferential surface of the opening of the container in a procedure of detaching the ink cartridge from the writing instrument body,
• the male fitting portion and the female fitting portion are fitted in the procedure of attaching the ink cartridge to the writing instrument body, and,
• in the procedure of detaching the ink cartridge from the writing instrument body, fitting between the male fitting portion and the female fitting portion is released.

A writing instrument, comprising:

• a writing instrument body provided with a pen point to be a writing tip end at a front end of a barrel;
• and a replacement ink cartridge provided with a tubular container having an opened front end and a closed rear end, wherein
• the ink cartridge has a plug fitted in an inner circumferential surface of an opening of the container and blocking the opening,
• the writing instrument body has a projection pressing the plug rearward when inserted into the opening of the container to release fitting between the plug and the inner circumferential surface of the opening of the container,
• the fitting between the plug and the inner circumferential surface of the opening of the container is released in a procedure of attaching the ink cartridge to the writing instrument body, and
• the plug blocks the opening of the container again in a procedure of detaching the ink cartridge from the writing instrument body,
• the writing instrument further comprising:
• the ink cartridge including the container in a bottomed cylindrical shape having the opened front end and the closed rear end and the plug fitted into the inner circumferential surface of the opening of the container;
• and
• the projection inserted into the opening of the container to press the plug rearward, thereby opening the plug of the ink cartridge, wherein,
• when the ink cartridge is connected, the press opened plug is in a state of approximately making contact with the projection and the plug is approximately clamped by the projection and a part of an inner wall of the container and its movement is suppressed, and,
• when the ink cartridge is replaced in a state of the opening of the container oriented downward, the plug moves by its own weight to be locked again in the opening of the container.