A writing instrument for a water-metachromatic member has a cylindrical container capable of holding water, formed with an opening at a tip end thereof to communicate the interior of the container with the exterior thereof, a hollow cylindrical holder which directly holds a pen point member, being formed at a tip end with an opening through which the tip end of said pen point member is projected outwardly and at a rear end with a communication hole, and a gap provided between an inner peripheral surface of the cylindrical holder and an outer peripheral surface of the rod-like body to form a gap for generating a capillary force therebetween. When the holder is held to the container, water is supplied through said communication hole to said pen point member. When the holder is detached from the container, the opening of the container is usable for water supply.