An ink filter having a fine mesh structure secured to an ink supply passageway is erected at a side portion of an ink reservoir. An upper portion of the ink reservoir communicates with the atmosphere through an air filter, a lower side portion communicates with the ink supply passageway. An ink filter interrupts the ink supply passageway at an upper portion and has a predetermined height to ensure maintenance of a column of ink in the ink supply passageway. As a result of the capillary phenomenon of the ink filter on the ink, the ink fills the entire portion of the ink supply passageway located below the ink filter so that the liquid surface of the ink is above the communication portion between the ink supply passageway and the ink reservoir but below the position of the ink filter.