In one embodiment, a data processing method is for processing data in a writing apparatus performing multiple writing by using multiple beams. The data is for controlling an irradiation amount for each beam. The method includes generating irradiation amount data for each of a plurality of layers, the irradiation amount data defining an irradiation amount for each of a plurality of irradiation position, and the plurality of layers corresponding to writing paths in multiple writing, performing a correction process on the irradiation amounts defined in the irradiation amount data provided for each layer, calculating a sum of the irradiation amounts for the respective irradiation positions defined in the corrected irradiation amount data, comparing the sums between the plurality of layers, and determining whether or not an error has occurred in the correction process based on the comparison result.