Pen core for writing instrument

In a pen core for a writing instrument having an ink conducting core for conducting ink to a writing member thereof and a barrel with an ink reservoir, said barrel being fitted on the outer wall of a pen core body, the improvement wherein:

• an annular groove in the form of a single continuous passageway is formed in the outer wall of said pen core body, said annular groove having a front end connected to an ink sucking groove which is communicated with said ink conducting core, and an air passageway having a front end communicated with an air hole and a rear end communicated with the rear end of said annular groove is formed in the outer wall of said pen core body without crossing said annular groove;
• wherein said annular groove has the form of a single continuous passageway formed in the outer wall of said pen core body, said continuous passageway being bent and turning in a labyrinth structure and leaving banks, and wherein said air passageway is formed in a longitudinal one of said banks.