1. A pattern writing apparatus, comprising: a storage device configured to input and store pattern data of a mask pattern of each of a plurality of sub-regions created by dividing a pattern region of a desired pattern that is to be formed on a semiconductor substrate, the pattern data is input from outside; a merge processing unit, configured by hardware, software for a computer, or a combination of hardware and software, to read the pattern data of the mask pattern of each of the sub-regions and to merge the mask pattern of each of the sub-regions to be rearranged according to the desired pattern to produce a merged mask pattern; a correction amount calculation unit, configured by hardware, the software, or a combination of hardware and the software, to calculate a correction amount to correct a pattern dimension error, by loading effect, in each of a plurality of mesh regions created by dividing the merged mask pattern; and a pattern writing unit to write the mask pattern corrected by the correction amount for each of the mesh regions in an independent separate position on a mask substrate for each of the sub-regions by irradiating a resist layer formed on the mask substrate with a charged particle beam.