Abstract: An oil-based ink composition for writing instruments is provided that is capable of improving storage stability of an ink, while maintaining a leveling property even when a soluble gelling agent is used. The oil-based ink composition for writing instruments comprises a colorant, a resin, an organic solvent and a gelling agent, wherein said organic solvent(s) is (are) hydrocarbons solvent(s), and in particular, at least one species of the solvent selected from the group of aliphatic hydrocarbons solvent(s), alicyclic hydrocarbons solvent(s), aromatic hydrocarbons solvent(s), and halogenetad hydrocarbons solvent(s), said gelling agent(s) comprises (comprise) soluble gelling agent(s) soluble to the above organic solvent(s) and insoluble gelling agent(s) insoluble to the above organic solvent(s), and the above insoluble gelling agent(s) is (are) urea-modified thixotropic agent(s).