An oily ink including a pigment, a pigment dispersant, and an organic solvent, the oily ink further including 0.5 to 7% by mass of resin particles, which are swollen by the organic solvent, wherein (1) a dispersion of 1% by mass of the resin particles in an equilibrium state of swelling in the organic solvent has an optical transmittance at 700 nm ranging from 20 to 70%, and (2) a dispersion of 10% by mass of the resin particles in an equilibrium state of swelling in the organic solvent has such a viscosity that a ratio of the viscosity to a viscosity of the organic solvent ranging from 1.5 to 5.0. The oily ink form dense printed images on plain paper.