A condition setting process (S2) of determining peening conditions for water jet peening on a peening target object, an analysis process (S6) of analyzing a jet when a liquid is jetted from a nozzle model to a peening target object model and obtaining (S8) a void rate that is a volume rate of air bubbles contained in a unit volume of the liquid and a collapse rate that is a volume of the air bubbles collapsing in a unit time in the unit volume of the liquid in each position on a surface of the peening target object model, a shock pressure correlation value calculation process (S9) of obtaining a shock pressure correlation value that is a product of the void rate and the collapse rate in each position, and a range calculation process (S10) of obtaining an effective processing range in which the shock pressure correlation value on the surface of the peening target object model is greater than a predetermined value, based on the shock pressure correlation value in each position.