Abstract: A data processing apparatus comprises a grid pattern area calculation section (24) for calculating the minimum grid and the present area of a circuit element for each layer of circuit patterns given by CAD data (1); an overlap area calculation section (25) for calculating an overlap area of present areas; and a composition/division optimization judgment section (26) for judging by a criterion whether the layers including the overlap area should be processed according to a single common grid or different grids. Each layer can be assigned the grid with the minimum accuracy required for the layer. A grid with more minute accuracy than it requires may not be used. Operation load in making reticle mask data and processing load in actually performing exposure or the like are thereby considerably relieved.