Disclosed is a method of diagnosing a conversion process for converting a format of image data including unit data corresponding to charged particle beams into a format suitable for an aperture array, the aperture array having a plurality of controllers provided to match a plurality of the charged particle beams to control the charged particle beams, and a driver configured to drive the controllers. The method includes: extracting the unit data having an identical first rank based on an arrangement of the unit data in the image data from the unit data of each block including a predetermined number of the unit data and calculating a first checksum of each of the first rank; extracting the unit data having an identical second rank after the conversion process from the unit data of each block and calculating a second checksum of each of the second rank; and comparing the first and second checksums.