An oil-based ink for a printing plate by an ink jet process comprising spraying an oil-based ink comprising at least resin particles dispersed in a nonaqueous carrier liquid on an image receiving layer of a lithographic printing plate precursor dropwise from a nozzle to form an image, and then desensitizing a non-image area of the image receiving layer by a chemical reaction to prepare a lithographic printing plate, the image receiving layer being formed on a water-resistant support and comprising zinc oxide and a binding resin, wherein the resin particles dispersed are polymer particles obtained by polymerization granulation of a solution comprising (i), (ii) and (iii): (i) at least one monofunctional monomer (A) which is soluble in a nonaqueous solvent which is at least miscible with the nonaqueous carrier liquid and becomes insoluble therein by polymerization; (ii) (ii-a) at least one monomer (B) represented which is copolymerizable with the monomer (A), or (ii-b) at least one monofunctional macromonomer (MA) obtained by combining a polymerizable double bond group with only one end of a main chain of a polymer comprising a repeating unit corresponding to a monomer; and (iii) at least one resin for dispersion stabilization (P) which is a polymer comprising a repeating unit, wherein the polymer is crosslinked at a part of its main chain thereof, and the resin (P) is soluble in the nonaqueous solvent.