An ink composition for a pressurized ballpoint pen and a pressurized ballpoint pen which can exhibit excellent writing properties, do not cause ink leakage and which are suited to handwriting. The ink composition for a pressurized ballpoint pen includes at least a colorant, a solvent and silica, wherein a coagulated matter containing at least the silica is contained therein, and the ink composition has a viscosity of 10000 to 40000 mPas at 25.degree. C. A pressurized ballpoint pen equipped with a refill, in which pressurized gas is charged into an ink reservoir charged with the ink composition for a pressurized ballpoint pen and a closed vessel containing the reservoir is also set forth.