PURPOSE: To reduce the capacity of a memory required for processing by enabling the transformation of a Bazier data signal for each processing range (such as 64 lines).

CONSTITUTION: The Bazier data signal is divided/carried over inside a Bazier data signal processing part, vector transformation is performed for each processing range after flatness is confirmed and further, intersection transformation is performed only to the vector data signal belonging to the inside of the processing range. The vector data signal outside the processing range is carried over to the inside of a memory for carry-over, and an intersection signal V4 is stored in one line buffer 11. The intersection signal V4 in the next processing range is stored in another line buffer 12. During this processing, an intersection signal V6 is read out for each line, outputted to a synthesizing part 18 as a change point signal V8 after sort/clipping processing concerning a main scanning direction Y, synthesized with an image data signal V9 and converted into a dot signal V12 later. Based on this dot signal V12, exposure on a photosensitive film 33 is performed.