To provide a novel pretreatment liquid, thereby to improve printing density, prevent strike through, and improve primary fixability of ink, in an oil inkjet printing system in which the pretreatment liquid is applied to a printing medium and thereafter oil ink containing a pigment and a solvent is ejected onto the printing medium, thereby printing is performed, particularly in an inkjet printing system performing printing on plain paper by using a line head type inkjet printer.

As a pretreatment liquid, a liquid containing inorganic particles having an average particle diameter of 1-20 μm and a solvent is used. The inorganic particles comprise 30-80 mass% of inorganic particles (P-1) each having a value of the following liquid absorptivity (A) of 0.2-0.6, and 20-70 mass% of inorganic particles (P-2) of a value of the liquid absorptivity (A) of 1.0-1.5. A=B/C (1), wherein A is liquid absorptivity of the inorganic particle, B is an oil absorption amount (ml/100g) of the inorganic particle, and C is a specific surface area (m2/g) of the inorganic particle.