An oil based ink composition for inkjet printer comprising a coloring agent and a binder resin in a non-aqueous dispersion medium, wherein the binder resin comprises a block copolymer having a repeating unit (a) corresponding to a monofunctional monomer containing an aliphatic cyclic hydrocarbon group having from 5 to 30 carbon atoms or a graft copolymer having the repeating unit (a) in the main chain thereof.