An oil-based ink composition comprising a colorant, an organic solvent and a polymeric compound, and optionally an alkylamine ethylene oxide derivative as a pH adjusting agent, in which the pH of an aqueous phase is from 5.5 to 10, when ions in the ink composition are transferred to water. This oil-based ink composition prevents corrosion of a nozzle plate and is improved in storage stability, and thus can ensure printer reliability such as ink-jet stability obtainable even after long storage thereof, and can withstand outdoor service environments.