a pattern data processing apparatus processes pattern data based on which a predetermined pattern is displayed in a display area . the pattern data has data corresponding to both said display area and a data calculating area outside of said display area , the display area being divided into a plurality of pixels . the pattern data processing apparatus comprises a flag data generator for generating first flag data indicating whether each of the pixels is located within a closed area of the pattern from the pattern data , a flag data memory having a memory area substantially corresponding to the display area for storing the flag data , read - out means for reading out the flag data from the flag data memory in a predetermined scanning direction , means for processing the pattern data inside of the closed area of the pattern by using the flag data read out of the flag data memory , and means for successively mapping the pattern data from the data calculating area into an outermost peripheral portion of the flag memory and for storing second flag data representing flags of the data in the data calculating area .