An electron beam exposure system comprises a pattern data generator for producing first pattern data indicative of a desired pattern of electron beam to be written on a wafer and second pattern data indicative of the number of repetitions of the pattern specified by the first pattern data, as a time sequential mixture of the first and second pattern data. The time sequential mixture of the data is sorted in a data sorting unit into a parallel data of the first pattern data and the second pattern data. Then, a discrimination is made whether the data is the first pattern data or the second pattern data, and when the data is the second pattern data, the data that follows immediately behind the second pattern data is transferred to an output path simultaneously with the second pattern data, which is transferred to another output path. Thereby, the first and second pattern data form a parallel data. The parallel data thus formed is next compressed by deleting the data, that follows immediately behind the data which contains the second pattern data, from both output paths.