In order to provide an ink reservoir for a writing instrument which has satisfactory permeation resistance and moisture-absorption resistance against permeation of a solvent and absorption of moisture and which is excellent in a storing property of the ink with the passage of time, an ink reservoir for a writing instrument in which an ink for a writing instrument is stored is constituted of at least one selected from the group consisting of a cyclic olefin resin, a fluorine base resin and a high molecular weight olefin resin or the ink reservoir for a writing instrument described above is constituted of a laminated structure which is formed by extrusion molding of two or more kinds of thermoplastic resins.