A method of evaluating the exposure property of data to a wafer in which errors in the production of a photomask and the formation of patterns caused by defocus in the transfer of data to the wafer are considered. Accordingly, errors in the production of the photomask and deformation of patterns caused by defocus can be evaluated in the stage of design data. Oversize processing and undersize processing are given to pattern data of the object of the process by figure operation for the whole pattern data within errors of the production of the photomask according to the specification thereof, and simulation is used as a reference to the original pattern data of the object of the process, the oversize data in which oversize processing is given to the pattern data and the undersize data in which undersize processing is given to the pattern data, in which the wafer exposure simulation is carried out under the condition of zero focus, or under each exposure condition of zero focus, a given value minus defocus or a given value plus defocus. The exposure property of data to the wafer is evaluated from the results of the wafer exposure simulation.