A water based ink composition for a ballpoint pen containing ι-carrageenan in addition to a dye, water and a water-soluble organic solvent. When used in the ball point pen, the ink composition satisfies an ink drooling prevention and a drying resistance which are mutually incompatible functions and is excellent in writing performance regardless of containing a dye as a coloring agent. The ink composition (c) is used, for example, by accommodating a refill (12) whose ink accommodating tube (18) is filled with the ink composition and which is further loaded with an ink backflow-preventing member (20) in the ballpoint pen (10) whose writing edge is retracted and extruded from a front end opening of a barrel (21).