PURPOSE: To obtain a pigment ink composition which consists of ultrafine pigment particles and has excellent long-term stability by dispersing a pigment in an aqueous or oil medium, followed by ultrafiltration or reverse osmosis of this dispersion.
CONSTITUTION: A pigment ink composition comprising a dispersion of a pigment consisting of particles of a diameter of 0.2μm or smaller in an aqueous or oil medium. An example of an aqueous medium, which will form a water-base pigment ink composition, is water or a mixture of water with a water-soluble organic solvent such as a glycol ether or glycol ether ester. Examples of oil mediums, which will form an oil-base pigment ink composition, include a nonpolar solvent, an alcoholic solvent, an ester solvent, and a ketonic solvent. If the particle diameter of the pigment is larger than 0.2μm, the composition will have inferior long-term stability.