in a writing instrument comprising a writing core and an ink tank , the contact of the writing core with the ink in the ink tank is intercepted so that unnecessary loss of the ink due to evaporation from the core surface and excessive supply of the ink to the core are effectively prevented . merely pressing the end portion of the outer case of the writing instrument allows a spring , a valve member and a flexible bellows portion of the ink tank to supply an appropriate amount of ink into the core .