Abstract: An oil-based ink composition for a ballpoint pen has an ink viscosity at 25° C. is 200,000 mPa·s or more at a shear rate of 0.019 s?1 and 5,000 mPa·s or less at a shear rate of 10,000 s?1 and a storage modulus at an angular frequency of 0.019 rad/sec is 10 Pa or more. The ink composition ensures a thin-width writing line, without bleeding or feathering, and prevents ink-depleted regions at the central part of the writing line so that the writing line is uniform and clear.

Abstract: An object is to make an ink composition for a ballpoint pen, ensuring that the width of writing line is thin, even slight unevenness or feathering does not occur at the boundary between the writing line and a paper sheet, ink-depleted writing line that is writing line in a state of ink being lacked near the central part of writing line is prevented, and the writing line is clear. The ink viscosity is set to 200,000 mPa·s or more at a shear rate of 0.019 s?1 and 5,000 mPa·s or less at a shear rate of 10,000 s?1 and the storage modulus at an angular frequency of 0.019 rad/sec is set to 10 Pa or more.