Disclosed is a process of preparing a planographic printing plate from a light sensitive planographic printing plate material comprising a hydrophilic support and provided thereon, an image formation layer containing (A) a polymerization initiator selected from an arene-iron complex and a tribromoacetyl compound, (B) a polymerizable ethylenically unsaturated compound, and (C) an alkali soluble resin with an acid value of 5 to 200, the process comprising the steps of imagewise exposing the light sensitive planographic printing plate material to ultraviolet light using an exposure device comprising a digital mirror device (DMD), and removing an image formation layer at non-exposed portions with an aqueous alkali solution to form an image.