A refill of a low viscosity water-based ink ballpoint pen is provided with a housing, a pen tip attached to a front end of the housing, an ink tank (4) formed inside the housing, an ink guide member guiding ink in the ink tank (4) to the pen tip, and an air passage formed between the ink tank and the pen tip in the housing and connecting the ink tank and outside of the housing. The inside of the ink tank is divided by partition walls (11) into a plurality of ink chambers arranged in the longitudinal direction, and the ink guide member is used to guide the ink in these ink chambers from the ink chambers close to the pen tip successively to the pen tip. A substantially uniform clearance (13) is provided over the entire circumferences between the outer circumferential surface of each partition wall (11) and the inner circumferential surface of the ink tank (4). A ring-shaped ink film formed at each clearance (13) is used to hold the ink in each ink chamber. Air flows into the ink chamber through the clearance (13).