PURPOSE: To obtain bit map data faithful to a pattern expressed by an outline by generating bits on the outline neither too much nor too less.

CONSTITUTION: A bit map data generator comprises a coordinate resolving power changing means 4 for converting a position of a segment between pixels containing a segment forming an outline to a pixel unit 1/N (N represents an integer), an X-coordinate memory means 7 for storing a position where a Y-direction segment of the outline crosses an X axis and data of a crossing form, and a dot generation judging means 10 judging whether or not dot is generated for the appropriate pixel on the basis of the data stored in the X- coordinate memory means 7. A relation between a segment of an outline between pixels on a system coordinate and pixels is evaluated by the positional relation thereof, whereby bits on the outline are generated neither too much nor too less.