An organic solvent-based ink composition for ink jet printer comprises a dye or dyes dissolved in a mixed solvent of a good solvent and a poor solvent, wherein (1) a solvent having a dye solubility of at least 4% by weight is used as the good solvent, (2) a solvent having a dye solubility of less than 0.5% by weight is used as the poor solvent, and (3) when the dye solubility for the aforesaid mixed solvent is a % by weight and the concentration of the dye or dyes in the ink composition to the mixed solvent is b % by weight, the value of a/b is within the range of from 1.0 to 3.0.
