PURPOSE: To obtain the mask pattern data processor which suppresses the graphic deformation by slit-fill processing and executes graphic processing with high accuracy.

CONSTITUTION: This processor has an information storage means 1 which stores plural pieces of previously prepd. pattern data on an X-Y plane, a slit-fill processing means 2 which reads the pattern data from the information storage means 1, decides whether a spacing of a prescribed width exists between the read pattern data and the adjacent pattern data adjacent thereto and executes the processing to fill the pattern of the spacing part when the spacing does not attain the prescribed width and a connecting information forming means 3 which forms the connecting information of the pattern data synthesized by one piece of the pattern data and the adjacent pattern data by the slit-fill processing means 2. The pattern information stored in the information storage means 1 is read again when the pattern data is subjected to post processing by a logic processing means 4. The pattern data is subjected to the post processing in accordance with the connecting information formed by the connecting information forming means 3.