To provide an oil-base ink composition for a ballpoint pen, the composition favorably changing colors of handwriting without causing transfer of the ink into a blank part of a sheet to contaminate the sheet even when the handwriting is rubbed just after written, and to provide a ballpoint pen housing the ink composition.

The ink composition includes a reversible thermochromic microcapsule pigment and an organic solvent as a main solvent having vapor pressure of less than 5 mmHg at 20°C, wherein the microcapsule pigment exhibits photochromism between a chromatic state and an achromatic state relating to a color density-temperature curve and has a complete achromatizing temperature T4in a range from 45 to 90°C. The ballpoint pen houses the above ink composition.

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